8800GT "OC" cards generally start in the range of 640 MHz and work up to almost 700. "Standard" cards are 600 MHz. "OC" cards vary from manufacturer to manufacturer, and some manufacturers have several "levels" of "OC" cards (ex. EVGA & XFX). Generally it's only worth a few dollars more ($10-20 maybe ?) to buy a guaranteed "OC" card vs. buying a standard card and OCing it yourself. Look for cooling system and other design differences to aid in your choice, as having to add an aftermark cooler will add cost you can avoid up front. The new MSI 8800GT with heat pipe cooler and double slot width for $200 looks to be an excellent choice IMHO. Wish I had waited just one more week before I made my purchases or else this would be my choice, esp. at that price.
8800 GTS "OC" cards range from 660 upward, with "standard" being 650 MHz.
To add yet more confusion, the SP's also get "OC'd" and they run all over the place, varying again by manufacturer.
It's a nasty business these days ...