You'll probably get lots of contradictory advice, so here's the first salvo.
The 6600s are indeed experiencing their share of driver grief at the moment, so if you want short term stability, the ATI card may be a better choice. According to most who have compared to the two, the 6600 has incrementally better video quality than the ATI, but both will give very good picture quality.
Unless you want the integration of MCE, XP is just as good but offers more flexibility.