I think _all_ commercial Intel and AMD CPUs are now 64 bit (maybe some low-end ones may not be, IDK).

I don't know how fast 64 bit increases performance over 32 bit computer, in reality that is.  Quad core compared to dual core gives a ~60%-70% If I recall correctly increase in speed (no, it's not 100%).

I'm actually planning on moving to Windows XP x64 - most apps seems to work on it (I'm still trying to get my TV Tuner to work, I did get it working once, but for some reason, I can't get it to work anymore).  All 32 bit apps that don't really perform anything "big" will work due to "32 bit emulation" (it's not really that).
There's 64 bit versions of most apps you should need.  I'm currently trying to find a decent RAMDisk for XP64 (none are free, it seems).