It works just fine for DX9 games, as long as driver support is there. And DX11 games are a minority.
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Any new games that really need any sort of optimizations via drivers aren't going to be DX9. If a new games comes out in DX9, it is already using old tech which current drivers are going to be more than capable enough for.
Drivers will work fine in XP with tweaked drivers/INF, same as people have been doing for windows 2000. I'm running Win2k on 8 cores, 990FX and Nvidia 550Ti. Check my sig.