If you already have a compatible legal copy of Window XP SP2 or 3, I would install that rather than Windows 7. However, if you looking to purchase a copy of Windows to use with Boot Camp, get Windows 7 Home Premium rather than XP.
Additionally, 7 will be able to offer you DX10+ capability if your hardware supports it. XP is limited to DX9. That's something to consider if you're playing newer games.
Kind of true... Microsofts official support is for Vista and above... But I was under the impression that someone with too much time on their hands had created code to allow directx 10 to run on XP.