Thought I might add my current settings, since this seems to be the GTX 280 thread 'round these parts.

I bought a Zotac GTX280 AMP! stock at 700/ 1400/ 2300

Right now I have 'er stable at 750/ 1525/ 2430, testing Furmark windowed 4x MSAA, 1280x1024, Xtreme burn. I let it run an hour and up the clocks.

The block is MCW60 rev.2 plus unisink: