Do you already own the GTX285s, or are you planning on buying them? Because if you're planning on buying them, I would probably suggest saving that $500 and getting 2x 5870 or GTX480s to SLI, that would max out your budget, though, as each card is around $500, though the GTX480s have recently dropped. Also, with the AMD 6-series right around the corner, you might want to wait for them to be released before purchasing, as they will undoubtedly drop the price of the 5-series cards.
Also, 2x GTX470s is a decent choice...Or maybe 3x
And, do you really need 2 Multi-optical drives? Otherwise looks good.
Here's a chart of relative GPU performance I stole from another thread:http://tpucdn.com/reviews/Zotac/GeForce_GTX_460_Amp_Edition/images/perfrel.gif