Two GTX 260 in SLI should be faster than any other setup, with the HD 4870 X2 following closely. The GTX 285 should be the slowest of the 3, but not by much in many cases. To avoid the hassles of a multi-card setup, I recommend you get a powerful single GPU, which falls into either a HD 4890, GTX 275 or, of course, the GTX 285.
GTX260 can be had for $199 - $50 rebate. Tiger Direct
HD4890 $229 - 20 rebate. Tiger Direct
GTX275 is about $249
Gtx285 is about $334 at buy.com
What's the best deal?