As sswilson said the best setup with your rig would be dual GPU clients, you need to have SLI turned off as F@H dosn't use it. For multi GPU's you need a separate client instance for each with different Machine ID'S, check this guide
Folding@home - WinGPUGuide. Each GTX 280 should be good for 7000+ PPD. Don't forget to download Fahmon to moniter your PPD...