I asked this same question here. This is what I ended up doing and I'm happy with how everything went, more or less.
When I was building my new computer a couple of years ago, I wondered whether to install Windows XP 64-bit. The majority here said it wouldn't be that noticeable, and some issues may arise. Basically, they said if you don't have or need more than 4 GB of RAM, it's probably not worth it. So I didn't do it (I had exactly 4GB).
Then, a few months ago, I was ready to install Windows 7. So many more people use 64-bit for Win7 as compared to WinXP, that it seemed like a pretty safe thing to do. So I did, and I'm happy. The most difficult issue I ran into was trying to get the Windows7 install dvd to boot and install properly because of some AHCI issues I had (which is in a thread here, and I remember somebody giving me fantastic instructions to resolve it). So that's my story.