Windows Vista x64 (SP1).
Although my computer came with only 2 GB of RAM installed, I think it's better to go with a x64 computer now if you want it to be some kind of "future-proof". It's just a matter of time for 32-bit to be abandoned, certainly in enterprise environments where servers need as much RAM as possible to run a lot of services. Of course, buying a 32-bit PC now may work for another couple of years, but in the end you'll be better of with 64-bit.
And I can still put more RAM in my PC when I need it! 
/ 
