> XP is *never* the way to go for performance; best performance for Windows is always
> achieved with 64-bit MAME running on 64-bit Windows 7.
The performance of MAME64 on Windows XP Professional x64 Edition is comparable to Windows 7 64-bit.
For people running real arcade monitors, XP x64 is the best choice, especially when running GroovyMAME with Calamity's modified ATI drivers (CRT_EmuDriver) and a compatible ATI video card (see my signature for links).
However, for people running LCD monitors, Windows 7 is the way to go.
GroovyMAME support forum on BYOAC