What sort of system were u planning on getting?
Imo id get 2 4870s with a Intel Chipset, but if its X58 then it doesnt matter.
I believe CF scales better than SLI does atm, but dont take my word for it.
Plus u can grab 4870s for $170 a piece. Pretty good if u ask me
What about the 4870 1 gb? It trades blows with the GTX 280 in some games (and loses to the GTX 260 in others). But from all the comparisons I've read, it seems the ATI cards are a better value right now (performance/cost). Anyway, just google 4870 1gb review, etc and you can see all types of perfomance comparisons.
The 2x4870 or the 4870X2 will outpower the GTX260 of course(if you can CF, but not on a 780i), but the 260 (216) is quite a bit better than the 4870 512mb and usually the 1gb version too. One thing about the GTX260 is it's a rather large card. It's 11 1/2" card I think, at any rate it's larger than my brother's 4870, and it comes quite close to my HD cage in my Antec 900, I understand why the power plugs are on the side now... I had to put the HD's in a different cage so the GTX260 would fit...
I wouldnt xfire or sli if you dont need to big hastle for little performance, dont xfire unless its a high end card, buy the single fastest card you can buy. Sounds like a 4870x2 or a gtx 295 is almost in your budget.