Mac Pro/Computer Games

So I am about to buy a shiny new mac pro in a few months but need some help with a few issues.

I am wanting to install windows with bootcamp to play games. Which version of windows should I buy. With so many versions out there which would be the best for games. XP is what I am thinking would be my best bet but exactly which one should I get?

I am thinking I am going to get the 2.8 quad with the standard graphics card. I can't justify spending another $140 for the NVIDIA because I don't think I would notice a difference being new to computer games. Will most games support both graphics cards?

Also when I go to buy games, given my eventually configuration (2.8quad, ATI, lods of RAM), what system requirements should I play close attention to. For instance as long as it says INTEL I should be fine right? ....or as long as it says ATI I should be fine? because the games are older than the processors and graphics cards they will not be on the back so does my logic/theory work when looking at the requirements?

Thanks

There are other reasons, Gaming is not one of the primary reasons or a huge issue but I am about to sell my 360 and would like to have a few games to fill the void every now and then. I am in college and have a job now so I don't really have time to play the 360 but would like the option to kick back and play evry once in awhile so I thought computer games on the pro would suffice.

My campus book store does not have Vista it has XP Professional w/SP2 for $92. It is odd that they do not have vista but yet they have Microsoft Office 08 in the store and it just came out.

...also I am actually going to use my ADC discount for the Mac Pro