Not sure of the fastest, but I run a GTX 460, it totally walked anything XP wanted to do with it and is still holding its own pretty well under Windows 7 even in new titles. Therefore you can probably cut the cost a little.
I never liked the 8000 series personally and found them all problematic, the 9000 series fixed that.
I think ATI supported XP for a long time, so cards like the 5770 probably worked under that OS and were fairly fast, along with the 58xx and 59xx I think. I don't know much about their line-up though as they had compatibility issues with some software I run that have never been fixed, not something you're likely to run into as this was a fairly specific usage case.
So yeah, once you get to the middle of the GTX 400 series it seems like you're topping out DX9 stuff anyway, it is even possible later cards won't be as fast as the hardware and drivers could be better optimized for DX10/11/12 which you wouldn't be running under XP anyway. Be warned, these things drink quite a lot of power.