One more geek-bit while I’m on a geek roll:
If you read around the web, you can see various prononucements that 64-bit Linux distriubtions aren’t up to snuff. Most authors seem to focus on the idea that the systems aren’t mature enough, or that they’re missing essential components that can make working with them difficult. When I read stuff like that, it makes me want to try.

On my new system (running an AMD64 3500+) , I’ve installed both the 32-bit and 64-bit versions of Ubuntu to compare. In the end, I found myself back to the 64-bit version, largely because it seemed more responsive on my hardware (particularly when I was, say, compiling a program, playing an MP3, and simultaneously doing a few other things). As for the bumps expected with 64-bit Ubuntu, they’ve been few and far between. The Ubuntu Forums provide excellent advice for solving problems, and with the awesome Automatix (yes, I know: everybody loves to hate Automatix, but it works) you can have a full-featured 64-bit system up and running in no-time.

Granted, I have found a few things that didn’t work perfectly, but they’re very minor, and all the big stuff works just fine for me.
So, if you’re like me and not sure, I’d say go with the 64-bit first and try it out for a day or two. I’ve had very few problems, and I find the responsiveness over the 32-bit version to really be significant on my hardware.