To get a decent experience with the type of games that you mention, you will definitely need to be using Boot Camp. This allows the Mac to run Windows natively without any of the slowdown associated with a VM system.

Case in point, I have a copy of Call of Duty, Modern Warfare 3 which runs at full everything using Win7 on Boot Camp on an iMac i7.
The same game, on the same iMac running XP under Parallels, is not really playable. It works fine on the opening screen but as soon as I move into the main street, where the action starts, the whole thing just slows to a crawl, and that's with everything turned down.

Given that information, the choice of Windows OS is moot as Boot Camp only supports Win7 and, maybe, above.

P