The 7900gs and 7900gtx cards are physically different, but you can overclock the 7900gs to gtx speeds. The gs doesn't have the same hardware as the gtx though.
You'll need to get a video driver from before Nvidia started using nTune. I think it's pre 94.xx drivers. Get the drivers from laptopvideo2go.com and also get the modified INF file. You'll then need to find coolbits to download.
Once you've got the drivers, and coolbits installed, you should be able to unlock the clocks in advanced properties for the drivers, and overclock.
I was able to get 3dMark05 from about 10,000 to 11,000 points with overclocking, but it did add a lot of heat to the cards. It's a big push for a small gain, so I run my 7900gs's at stock speeds for longevity...