I just upgraded my old (3+ years... I guess that's old these days) 32 bit single-core Celeron PC to a 64 bit dual-core AMD X2 processor, along the way taking the opportunity to pick a replacement motherboard that does all the video, LAN, Firewire etc. things for which I was previously using extension cards. No big news, except that I'm happy to report that no OS upgrade was needed: my 32 bit Ubuntu Gutsy has neatly popped itself into 32 bit compatibility mode, so I don't need to do a major software reinstall at a busy time. Cool!
The only thing that was a bit of a faff was getting the video output in the right mode, and that was entirely because of nVidia's binary-only driver distribution --- Ubuntu needs to be prodded (via the GUI) to enable this driver since it's not an open source compatible way of doing things. Otherwise everything is perfect and it's nice to have the two processor load trails appearing in my system monitor when I compile things now :-)