XP 64 aka Server 2003 was specifically designed for servers. Running it as a PC platform it tends to be rather slow and awkward. Most programs get put into the 32 bit (x86) program files anyway.
As I said I don't know about Vista but it was not designed as a server platform.
There is a Windows 7 thread here:
http://www.d-a-l.com/help/windows-vi...-7-beyond.html
Windows 7 will also offer both 32 and 64 bit versions.
For gaming the biggest leap is in DirectX. Windows 7 goes to DirectX 11.
Most cards today are DirectX 10 so the hardware may have to catch up here.
The difference between DirectX 9 (XP) and X 10 (Vista) is very noticeable so one can imagine the next level.
I have tested Crysis and Bioshock on Windows 7 and both run and look great!