There's still a potential hiccup with that though: the hardware.
Let's say you have a very advanced video card that handles Crysis 2 like a champ.... but then, oops, the Windows XP partition doesn't recognize it. Or perhaps the actual games don't.
Then again the idea mentioned elsewhere of just having two Win7 boot partitions might work.
Still, if I were the OP I would instead build an XP machine for games from 1999-2006 then worry about later games.... well... later.