I'm not quite sure what you mean by "signal strength." Could it be that you're trying to connect to your router through a wireless connection (a.k.a. "WiFi" Ethernet) and that you have poor signal strength? Or could it be that you have a poor signal from your ISP?
If it's "WiFi" (802.11a/b/g/n) then you may simply want to reposition your access point which is very likely also part of your router.
But if you're having poor signal strength with your ISP then you may want to call them and have a technician come out.
But as far as flashing a router with new firmware goes, about the only advantage Tomato, DD-WRT, or any of the other firmware alternatives will give you are possibly a few more options like the ability to slightly increase WiFi signal. It's good stuff and definitely worth looking into especially if it adds better encryption like the ability to use WPA2 - which would probably be about the only real reason to seriously consider flash upgrading. But flash your router (or anything) wrong or interrupt the flashing procedure and you could end up with nothing but a pile of worthless plastic (a.k.a. "bricking" your device).
You probably just need a better "access point" which usually means getting a better WiFi capable router. And then maybe choosing a better location to put it in too.
You also probably don't need a 802.11n-capable router either. Don't misunderstand! "N" is good stuff but both your router (or access point) AND your PC/tablet/or whatever have to have it. "N" is great for wirelessly transferring files or streaming content within your own home/business LAN. But "N" is a bit of a waste of money for the average Internet connection. You can probably get by just fine with older 802.11g. And if all you ever do is share an Internet connection then you can probably get by with even older 802.11b (although you probably can't use WPA2 encryption with older 802.11b stuff).
I could go into IPv4 vs. IPv6. But right now, and for a considerable time to come we will be using IPv4. Your ISP may not even support IPv6 even though some places actually have it. But so far, not a lot of adopters. So I won't try and confuse you. IPv6 is not likely something very many people need to worry about just yet. I just want to tell you about it in case you have an ISP who fully supports it or for some other reason you need to be aware of it.