I can comment on GTX 260 overclockability.
Yesterday I started testing my new BFG GTX 260 OC card, and I went straight to 702/1458/1188. Rock stable, max temp under load is 66C. I didn't try any higher clock, but I will...
The only benchmark I have for now is 3DMark06, ~16,500 (~6,600 SM2.0...