Well most people will agree that XP is better for gaming since it uses less resources, however Vista is the only OS (so far) that supports DX10, and a few of the new Microsoft games only work on Vista.
I would recommend Vista Home Premium or Ultimate 64-bit.
Isn't it obvious? XP is currently better because most current games were designed for XP. You'll notice a decrease in FPS if you play the same game on Vista. But this will shift in the future, when games focus on Vista...