the geforce 7900gs and 7900gt have been discontinued for a while so take them off your list.

if your power supply is 350w (or less) or you want to keep the price tag under $125, then this geforce 8600gts 256mb for $110 (before $20 rebate) would be your best upgrade.

if you've got a 400w power supply and $150 to spend, look at this geforce 8800gs 384mb for $150 (before $30 rebate). the next step up is a geforce 9600gt 512mb at $175.

if you want/need a better power supply, consider this antec basiq 500w for $50.