Generally, you only need to use 64-bit OS's if you're going to be doing something that utilizes more than 4GB of RAM (like heavy duty video editing, etc) because that's the limit of 32-bit OS's. I think 64-bit distros are getting good enough that there's not as many application problems, but it used to be a royal PITA to get flash, video, etc working in 64-bit.
I have a 64-bit machine that runs 32-bit Debian. I may be missing out on some performance, but at the time 32-bit was a little more stable. And since Debian upgrades so well from one version to the next, I'm not sure when I'll take the plunge and reinstall as 64-bit (maybe not until I build a new box 4 or 5 years from now)...