Every card you mentioned will do what you want. The 9600 is maybe a waste of money. A 4830 will beat it in every game for $80 - $100. The 4850 is amazing for the money.
Here's a rough performance breakdown:
Nvidia 9800 GTx >= ATI 4830
<= ATI 4850
< ATI 4870
If you run 1600 x 1050 or lower resolution there's really no difference in any of the the cards in terms of playability. The 4870 is certainly the best by a wide margin.
The 4670 should be mentioned as well. 4670's in cross fire do surprisingly well in the benchmarks. 4670's are $70 and 4830's are $90. The $20 is worth it.
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