I'd suggest this eVGA board instead. It's $70 less, supports three cards, and gets better Newegg reviews.
EVGA E758-A1 3-Way SLIhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813188039
As for the video cards, I'd try one GTX 285 for now. Add another if/when needed. Add a third only if really necessary. Currently GTX 285 Tri-SLI is the fastest thing available. It even beats two GTX 295 cards (Quad-SLI).