I've had pretty decent luck OC-ing my first GTX260, it was a 192SP Vanilla EVGA. It would get up to 756/1556 1300 iirc. Or close to it from a stock 566/1242 1000.
I will say it again and again, from what I've found from a few major GTX MFG's, the Pre-OC'd versions have nothing different but CLOCK speeds. With the new 216SP versions, I noticed it's running lower voltage for 3D (DVD/Movie/HD) and EXTRA (3D Gaming). My original card ran at 1.12v for those settings, my new 216SP version runs at 1.06v. I've yet to OC my new card beyond GTX260 FTW clocks, as I don't need it, but my temps are a few degrees cooler. Also 75A% fan speed is a sweet spot I've found for noise/cooling performance.
8800GTS 512's are great cards I know (not from experience), and OC pretty well. Still pretty damn powerful for many today. I'm actually installing one in a friends' build that I got for $99 off of EVGA B-Stock...it should be here Tuesday, you better believe I'm gonna play with it!