7300gt beats 7600gs, and also the x1600 in some benches.
I faced the same choice, i picked 7300gt. Look up some benchmarks and you will see the difference. If your over clocking the 7300gt also makes the gap much larger, increase performance +30-35% (no the GS does not overclock as well as the 7300gt).
Make sure you get a ddr3 version though, and the 512mb i have is marginally better then the 256, so dont bother with that.