Someone got a 7900GT upto 800Mhz core using a massive copper heatsink with a 120mm fan... i'll try and find a link for it.
STICKY OR SOMETHING ON THIS PLZ -> http://www.vr-zone.com/?i=3437&s=2
The 7900GT IS a 7900GTX, the only difference is it comes with a smaller stock HS and is UNDERVOLTED. With a 'simple' voltmod and some aftermarket cooling, you can have 7900GTX performance! (Althought with only 256 memory instead of 512, but hey, you can complain about a 80% OC!)
link for the volt mod -> http://sg.vr-zone.com/?i=3330&s=2
This link also has a volt mod for the 7600 and the 7900GTX.
I know i WONT be trying this on my 7900GTX. Ill be damed if i risk a $1000 piece of hardware for a bit of performance. It'l prolly be bottlenecked anyway.