If you can find one, then i would tend to agree with xyzionz, the 4770 may be your best bet. From what i understand (though i may be wrong) the newer 40nm-whatever will run cooler and take less energy than the current stuff. The 4770 will give you better performance than a 9600gt and the 4770 will ideally only get better as it is new technology that will have new drivers released as time goes on.
personally though, i use an MSI 9600, the red one not the ugly black one, and it does fantastic. I can play Fallout 3 will everything maxed out. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127333
~i would really look into the 4770 for your application