I'm not going to flame, but the 4870's are way better value. You can get two of them for the price of a single GTX 280
GTX 280's are a bit faster (~10 maybe 15% with current drivers), but nowhere near faster (%) than the price difference (%).
You could even get a single 4870 now, and crossfire it with a 4870 X2 when they come out. You'd get higher performance than two GTX 280's and pay less money.