most 7600GS's can do 600 on the core with after market cooler but few ever break 900mhz memory. A few people have gotten to 1ghz though but not enough to hang your hopes on it.
Bought the low profile MSI HDCP one for a friend and got it to 540/900 with stock cooling/volts but the ram gave me the big stinky at 910mhz.
Oh, if u like AA and higher resolutions then the X800GTO will still be your buddy vs 7600GS, not to mention the supperior AA quality as well though video playback quality is better on the 7xxx series : /
And there is no GDDR3 7600GS, the 7600GS is a different GPU than the 7600GT, it is not an underclocked 7600GT, it cannot even support GDDR3 at all. So you probably won't get very good results overclocking the ram for higher memory bandwidth which is a good part of the 7600GT's strength. And before anyone tries to name Gigabytes 7600GS for having GDDR3 know this, it's not a 7600GS, it's an oc'd 7600GT rebadge.
To retor to the below reply on the "EDIT : on newer games"
Not much is going to be done with $70 ish for newer games to start with. So might as well get a good card that does decent on low res on new games and excellent on high res at older games. That would be the X800GTO.