I'd be curious to see how 64 bit works for you. Certain things, like flash, had poor support in previous versions of 64 bit Ubuntu. I haven't kept track of how its coming along as I've always run the 32 bit version.
I'm running 64 bit Ubuntu 8.04 on a new Shuttle bare-bone system I just built, with an Intel Core 2 Quad CPU. There is a trick to getting flash to work, but it's well documented, and with a little searching on the Ubuntu forums I found the fix. Everything else seems to work ok, including 64 bit VirtualBox, which I use mainly to run 32 bit windows 2000!
I know I've posted before that I haven't had much luck with Ubuntu, but 8.04 seems to finally be polished enough for me to use everyday, and it's replaced PCLinuxOS on two of my computers.