A few questions...

well here is how I would handle this. I would use a program called netstumbler to check what channels my neighbors were running at. There are three main that don't interfere 1,6,and 11. Most routers default to 6 so if you don't know 1 and 11 are good guesses. As for throughput Airport expresses are great with WPA encryption I get 12 mbps through almost all my house with two of them in WDS mode. You could probably get more with WPA2 encryption (it's more efficient generally). As for whether having two computers will affect your gaming this primarily depends on your internet or file transfers between computers. If your transferring files between computers or downloading on one computer while playing on the other it might effect it.

As for the vista 64 bit sry outside my expertise. I assume anything with a an atheros g chipset would work since they work for linux, mac, xp, ect. Personally I avoid vista like the plague. I would much rather run ubuntu with windows xp on a virtualbox virtual machine for my engineering apps. But for gaming I guess it going to have to be xp or vista. Hope this helps you out.