Definitely a 7600GT over a GS, much better performance. If you're going for 2 7600GS's though, I'd rather get a single 7900GT, except the ASUS version is a bit more expensive than 2 7600GS's. If you can accept a different manufacturer than ASUS, XFX or EVGA for example, you could get a 7900GT for the same price as 2 ASUS 7600GS's.
Although, if you're willing to sacrifice the possibility of SLI for a while, an X1900XT
would give you performance way beyond these cards for quite a decent price, $240. Listed above is
a 256MB version, but the extra 256 found in most of the X1900XT's won't improve performance much more.
Those are what I would recommend. If $240 is out of your reach, you could just get a 7600GT. But trust me, 1 X1900XT would be way better than 2 7600GS's. But you can't do SLI with ATI cards on a SLI-ready board, so it's your decision. Between your two, definitely the 7600GT.