GTX 260 performs within 10% of the gtx280 - just like the 4870.
If anything, Nvidia should be ashamed of the gtx280 - but the gtx260 is a great card and has great SLI performance. The 4870 is the better single card, but as far as dual card situations go, the gtx260 is certainly comparable to 4870 CF.
I don't really understand how or why, but there is something very particular about the gtx280 which holds it back in SLI - Marvelous would probably be better able to understand this than I can as he has shown some interesting information about both the gtx200s and the 4870s in terms of how they perform fillrate. But from what I can understand, the gtx280 has screwed up fillrate calculations, something architecturally holds it back.
Anyways, the point is that the gtx280 is just not that great of a card; its performance is shadowed by both the gtx260 and the 4870, and SLI gtx260 and CF 4870 beat the living crap out of it for a very manageable cost premium. The fundimental problem with the gtx280 is that doesn't have nearly high enough of a jump over its little brother in SLI performance.
If you want one big GPU - then the 4870x2 is far and by the best solution you are going to see for likely quite a while. If you can't wait and want 2 gpus, i'd be relatively compelled to suggest 4870 CF simply because of better motherboard support. If you own a SLI motherboard already though its highly unlikely that anyone would find gtx260 SLI performance disappointing.
Anyways, thats my 2 cents - i'd recommend AMD products this time around, they deliver a good value/performance ratio and that is generally what matters to most people; regardless of platforms or math or any figures we throw at the discussion.