Personally, I'd use Vista x64 rather than XP Pro x64, because Windows XP is not natively 64-bit and the conversion of the kernel to 64-bit introduced a fair bit of instability and other issues. While XP may be marginally more efficient than Vista, if you want a 64-bit OS, go with Vista. It has far fewer issues than XP Pro x64.

And you should definitely pick up another 2GB of RAM. It's dirt cheap right now and it will improve render speeds in more complex scenes.