So, if all you care about is speed, XP's going to be your bet, at least for now. Vista's a mixed bag -- I like the better security and admin control, but it's inconvenient for a lot of people. It's also going to be your bet for DX10 for now, since MS seems unlikely to try a modification to XP to allow for the DX10 driver model. Also, you're going to have some driver problems on Vista still, at least until people convert to it, but that's true of 64-bit as well. Really, though, this hasn't hit me much.

Personally, I like Vista on the machines that have it, but I have no desire to convert the machines that don't. Some people run WoW on Linux.