I've got both XP and 7 (and Windows 2000 as well) and XP had felt faster (and 2000 fastest of all by far), however they've put in an effort to optimize operation with 7 and it now doesn't seem to have a speed disadvantage. OTOH performance with Windows 2000 has deteriorated as they don't put in any effort at all for that OS.
Like dealing with OS X, it's probably best to use the latest OS version. Alas, 8 is really ugly though.
The 64 bit architecture is more efficient than the 32 bit architecture, so if you Windows 7 is 64 bit it probably will outperform 32 bit XP. (I've got 32 bit Windows 7 because I was concerned about compatibility.)