What old games do you want to play? 64bit XP will work with more than 3 gigs... but:
So far I have had no compatibility issues with win7. I'd suggest moving on from XP. DirectX 10 was a few years ago and that won't work on XP either. You'll be stuck on old software with new hardware. Why even have 6 gigs of RAM if you're using an OS that was designed when 512MB was considered to be a lot?