Yeah, it would have to be a highly overclocked and/or modded X1800GTO to beat a 7900GS. The GTO at stock speeds trades blows and is basicaly equal to your 7600GT.
And while normally I would not recommend such a small upgrade (7600GT to 7900GS), it depends what you can get for the 7600GT. Best Buy has a 7900GS for $150 no rebates, so it might not be that expensive an upgrade. Still, I would only do it if the 7600GT is failing to please you in a game/resolution/setting you want to game at.