Sorry, this is a long reply, read to the end and it should be worth the effort. :)
Putting your router in a more central location in the house would definitely help the other areas of the house, at the expense, of course, of decreasing the strength of the signal in your office. Locating your router on the top floor AND at the corner of the house means very little of the wifi transmit signal power is going to much of the house. If it's really located in the upper corner of the house approximately 3/4 of your wifi signal is going outside into the neighborhood and not doing you much good inside the house. Plus, I've noticed that placing many routers up high tends to rob devices located below them of much signal. I think many of them are designed to transmit their best signal at the same height as the router in a circular pattern -- thus the reason most of your signal is probably going outside at the second floor level and being wasted on the neighborhood.
One thing I would try is relocating your router to a more central location on the first floor near the ceiling (perhaps on a high shelf or bookcase). Then test to see what sort of signal you get in the other parts of the house (upstairs and downstairs). Of course, this might require that you run some Ethernet cable from wherever the cable-modem or DSL-modem is located. It's okay to separate the two, you just need a nice long Ethernet cable! :)
If you have an Android tablet available, like the second generation Nexus 7 (sorry this won't work with an iPad or iPhone), you can use it to get a very good sense of your router's signal reach and the level of the neighbors' signals you are competing with. There's a great little app (free) called "Wifi Analyzer" by "FARPROC" on the Android store that gives you a real-time display similar to the one that inSSider does on your PC. This thing is much easier to use for testing your setup than inSSider because you can walk around the house with the display in front of you and get a better of sense of where the interference is and where the signal is dropping. You'll need an Android device that receives on both 2.4 GHz and 5 GHz channels to be able to see everything. The first generation Nexus 7 tablets only support 2.4 and many early Android phones also only support 2.4. Note that you need to switch between monitoring 2.4 and 5, you cannot see both at once on the app's display, but it's a very fast and easy switch (touch the upper left of the real-time graph and it will switch between 2.4 and 5 if you're device supports both).
Ultimately, finding the best location AND the best channel (1, 6 or 11 on 2.4) for your wifi router is a bit of a try-test-try-test... cycle. There is no perfect solution, especially if you have only one router to work with.
As others on this thread have already noted, 5Gig wifi channels are much less crowded AND they do not travel nearly as far as 2.4Gig signals in the house. In general, the higher the frequency the more energy it takes to transmit the signal over the same distance. 5Gig channels aren't less useful, they just tend to be more useful for more special needs -- like a dedicated channel for your home office laptop that needs to connect over a VPN or for a Netflix device or similar in the family room that only connects over wifi. Otherwise, your phones and tablets and casual PC email/net browsers should get by just fine using a 2.4Gig wifi channel.
I currently live in a two-story house (3100 sq ft) and ultimately ended up running some Ethernet cable through the attic and a few walls to get better wifi coverage in my house. If you end up doing something like this you can also run some wire to provide some "hard-wired" Ethernet taps in a few key locations so things like your office can just connect using a direct Ethernet connection, rather than wifi. To date, I've done this in three different houses I've lived in, it's a bit of work but in the long run it's well worth the effort. :)
Anyway, what this means is that I actually have MULTIPLE wifi access points located throughout the house. Two on the upper floor and two on the lower floor, located at each end of the house (this house is long and narrow). If your house has a fairly square footprint, like many newer homes, you could probably get by with just two wifi routers, one on the lower floor and one on the upper floor, probably offset from each other.
This is where the Wifi Analyzer tool comes in to play. To figure out the best placement for one or more wifi transmitters in your house, you can disconnect your router from the network and just locate it somewhere else to see how the signal fares in that new location. "But what about my Internet connection?" you ask. Not important for testing, the router will happily transmit it's wifi signal, you won't be able to do anything with the Internet while you're doing this, but you'll be able to see how locating it in another spot affects the signal by using the Wifi Analyzer on your Android tablet or Android phone (I recommend using a tablet if you have access to one because the antenna in the tablet is bigger and does a better job of distinguishing the various signals you're picking up.)
Once you figure out the best location for your router, you'll want to figure out the best channel to use. This gets tricky, because it depends on your neighbors, so a lot of the variables are out of your control (also one of the reasons I ended up flooding my house with multiple access points). On the 5Gig channel there should be several free ones to use, pick anyone and use it. On the 2.4G side it will be crowded. In the ideal scenario you should only use 1, 6 or 11. (There's lots of explanations out there as to why; trust me, I've experimented with it and had many conversations with my brother who is on the IEEE wifi standards committee -- 1, 6 and 11 are the best choices.)
When choosing between 1, 6 or 11 in the 2.4G space I recommend the following, in priority order:
1.) a channel that is free of other signals (perfect and ideal solution)
2.) a channel that has a very strong "overlapping" signal ("say what!?")
3.) let the router find the best channel
4.) pick the one that has the least number of "overlapping" non-comformists (see below)
Number 1 is obvious, if you can get a channel all you to yourself take it! Of course, as your neighbors change or update their equipment your free channel might disappear. :(
Number 2 is not so obvious. Choosing the channel that overlaps with your nearest neighbor (the neighbor with the strongest signal invading your house) seems counter-intuitive, but it works. The problem is that you are sharing these channels (you're sharing the wifi radio waves), whether you want to or not. It's better to share with a strong signal than with a weak signal; because when two (or more) overlapping routers and devices see each others signal they will negotiate access to the wifi. Trying to negotiate with a spotty low level signal results in lots of bad transmissions and retries, ultimately causing your throughput to go through the floor.
Number 4 means try to avoid a channel that has overlaps from people who have configured their router to center on something like channel 4 and 8 (in practice, this may be difficult or impossible to achieve). There is a popular theory out there that you can actually put together an array of wifi routers that use 1, 4, 8 and 11. There are also some very good explanations about why that doesn't work. The explanations regarding why this doesn't work are very hard to follow, but they are correct. Just for grins, since I have four routers in the house, I tried it for a week. My throughput was AWFUL for that week. The 1, 4, 8 and 11 theory is bogus, don't subscribe to it!
Number 3 is there to keep you from getting hung up on number 4. :) Also, near a microwave it's best to use channel 11. And, if one of your neighbors has turned on the ultra-super-uber-fast-turbo mode on their 2.4G channels their router is trying to use TWO of those precious three channels on the 2.4G spectrum. Figure out who they are and ask them to turn off the turbo mode. Most devices won't use turbo mode and it really raises cain for everyone nearby. Not to mention that their actual Internet connection is probably not even close to that fast! One way to avoid trouble with such a neighbor is to stick to channels one or six (one is the best choice -- those turbo things grab the next channel to the right when they kick in, so one tends to be more predictable).
Hope there's something useful in this rambling. :) Good luck.
p.s. I didn't explain how I actually configured multiple wifi routers to work on a single home network, it's not obvious or for the faint of heart. I recommend you find the best location for the 2.4G signal on your router and, if you're lucky, you'll be able to use the 5G channel for a dedicated purpose (like Netflix or your office network connection).