DX10 cards will work fine on xp, however they will only be able to play games in DX9. However nearly all DX10 cards are much quicker than DX9 cards and so will be of great benefit in games.
Most people who have dx10 cards still run games in dx9 (either in vista or xp) because at the moment dx10 is slower and gives little or no visual improvements over dx9.
As far as pcie 2.0, it doesent make a big difference yet, but your getting it for free and it is 100 bercent backwards compatible with a pcie 1.0 slot, so go for it
------------------------------E8500,GA-EP45-UD3R, 8 GIG MUSHKIN, XFX 4890 , ASUS 22", WD 640 X 2, CM 532, CM 650TX
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