You absolutely need windows 7. There is no reason whatsoever to even buy either of those video cards without it. Both are DX11 video cards and most games now are DX11 games. So get a $170 card and windows 7 or else you'll have something that you can't game on. I'm not even sure if those cards even have drivers for windows xp anymore.
GET WINDOWS 7 ABOVE ERRYTHING ELSE