XFX is a good brand, eVGA's Rival actually...
as to XP, they Just released Service Pack 3 for XP a week or two ago, but there was no DX10 in it. as far as I know, M$ has no plans or intentions to release DX10 for XP. would be kind of hard to do it anyways, since DX10 has some features that do away with some of the DX9 components etc etc...