OK, so you can't just add another GTX 280, with that MB and PSU. Just checking.
There are lots of benchmarks for HD 4870 CF and I'll use some to approximate the HD 4870X2. It should be very close.
The 4870 CF beats the GTX 280 in some games/resolutions (e.g. World in Conflict 1600x1200), but loses in others. To make it even more weird, look at Crysis 1920x1200. The GTX 280 wins by a negligible margin with 0xAA but the HD 4870 CF wins by almost 5 fps (16%) with 2xAA.http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/9159-ati-hd4870-hd4850-crossfire-performance-review-10.htmlhttp://www.hardwarecanucks.com/forum/hardware-canucks-reviews/9159-ati-hd4870-hd4850-crossfire-performance-review-14.html
Anyway, the GTX 280 and HD 4870X2 will be pretty close in performance.
I would make the switch from GTX 280 to HD 4870x2, but only if I could get a good price for the GTX 280.