I'm through with XP -- regardless. I don't even have it on my system as a dual-boot anymore.
Windows 7 is not gagging on the old games. Seems to run old games better than Vista did.
DX11 (and beyond) games should run faster for a given level of graphics goodness because of the DX11 features -- e.g. tessellation.
I think XP had less overhead than Windows 7. Thus: Older games -- well, one has to read the comparisons. I think old games may run a bit slower on Windows 7 than XP -- for a given system. However, our systems are so far beyond XP now that old games run faster on Windows 7 than they did on XP with the XP era hardware.
Time has passed XP by. In my opinion, of course

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