If wired internet is an option for you, I would seriously consider wiring the house with CAT5e or even CAT6 cable. Seeing as it is a new build, it may be too late to add it to the plans but it is also something you can do as a weekend project if you're so inclined and savvy enough.
If wireless is the only option, I would go with either a Wireless N router in the middle of the house on the bottom floor, closer to the ceiling if possible, and a wireless extender/repeater upstairs.
If you want to go the Apple Airport route (I've recently had a lot of success with setting up and implementing these devices in homes), you can get an Airport Extreme base station for downstairs and an Airport Express for upstairs to extend the network. Both options would work for what you want/require.