XP x64 is far and away the best OS MS has ever produced.

Its fast, its stable, it has all the important 64bit features. The ONLY thing it doesnt have is DX10, and the ONLY reason it doesnt is so MS can rip you off for 300$ for a Vista that was designed to please Big Media, and Big Hardware. (ie: it runs like shit).

/rant off

But yeah, XP64 is the way to go.