Unless you have a very specific program that doesn't run under x64, that's the way I'd go. Aside from other benefits, at least you'd get to use all of the memory you have. Despite what others may say who still think Windows 7 is like Windows XP, that extra memory can be usable to cache and prefetch, making your system more responsive....even if you aren't necessarily killing the system with intensity.
Basically, my thought would be x64 should be the default install, unless you can come up with a good reason not to. At one point, there was a rumor that Windows 7 would be x64 only, so why not use the current platform, than cling to the past. If you had 2 GB of memory, or had a unique app that wouldn't run but was absolutely necessary to you...then I'd say to go with x86.