No questions asked, go with the 6600 for sure. 4 more pipelines and SM3 (as oppose to SM2 on the Radeon). The Radeon is just older too. Remember that the Radeons were at the same level and competing with the GeForce 5s, so obviously a 6 series card would blow away any 9xxx card. If you had shown me a 5700 vs a 9600, I would have definatley told you to go ATi, but that's not the case. ATi kicked so much Nvidia arse back then with its 9xxx series.
I still reccomend that you shoot a bit higher though if you want to keep up to date with your gaming performance. Go for a 6800GS if you can.