Not sure what it will overclock to,but if you use coolbits,you should have no problem finding the cards limit.Use the older interface instead of the new as it is easier to work with for novice users.Try looking at 7900gt speeds and see if they'll work for your gs.And do the same for the 7900GTX,check its specs and see if your card will clock to that speed.You can also see about flashing the video bios to a GTX,but I would recommend using RIVETUNER to see if there are any extra pipes you can open.Highly unlikely but worth looking into.Goodluck.
Dahak
AMD X2 5600+ @ 2.8ghz(stock)
M2N32-SLI DELUXE MB
2 GIGS DDR2 800 RAM
THERMALTAKE 850WATT PSU
7950GT KO(WAITING FOR MY OTHER TO COME BACK FROM RMA)
ACER 22IN. LCD
SMILIDON RAIDMAX GAMING CASE
80GIG/250gig SATA2 HD's
XP MCE