I would stick with Nvidia definitely. I was recently shopping for a card as well, and from what I saw ATI was lagging in the budget department for quality.
The only thing I don't like about the 7600GS is it's a 256MB card with only a 128-bit memory interface. On the other side of the fence, something like the XFX 6800XT has only 8 pipelines compared to the 7600's 12, but the 6800XT does have a 256-bit interface and GDDR3 ram instead of the 7600's GDDR2. That can be very important, given your extra memory will be crippled by the bottleneck with a lower interface. I'd suggest finding some benchmarks of each card, as sometimes being in the newest series doesn't necessarily indicate a better card.
The 7600GS has a 50mhz faster core cpu, but 200mhz slower ram. The 7600GS is cheaper at $105 compared to the 6800XT's $115 after a $15 rebate.
The big deciding factor might be your PSU. The 7600GS requires a 350W, the 6800XT requires 450W.
I'd also suggest buying an eVGA card if available. They have a Step-Up program that lets you essentially return your card within 90 days and gets its purchase price deducted from a new card of your choosing. That's only a one time offer, naturally.
Edit: I should note, I was comparing the XFX 6800XT and the EVGA 7600GS. You can find a $96 Gigabyte 7600GS. There's even a BFG 3D Fusion one for $88 with a $20 rebate bringing it to $68. But that seems awfully low to me.