I had bought HD4870's this weekand and a Gigabyte X48 CF mobo, and trust me, GTX280 SLi is way, WAAAAY faster in COD4 and in GRID, the 2 games that the ATi's should be blazingly fast in. I tested the CF setup very thoroughly.
Well, I found out the Radeon Red cards actually sucked in these games comparing to 2 GTX280's in SLi.
Stuttering, framedrops, a lot
of heat coming out of the back of the card idle, a frying hot PCB and memory. Over 100C in CF. Bad IQ. Very noisy at only 35% fanspeed, and 35% fanspeed is not enough to cool the cards properly, so I think these cards have to be clocked down in 3D to be cooler.
Well, I think (actually tested it myself) the radeon HD4870 sucks!
I tried 8.7 beta and 8.6 hotfix, and I tested if Crossfire was working, and it was, but it was a lot, a lot
slower than GTX280 SLi.
I cannot believe 2 GTX260's in SLi are slower than 2 HD4870's in CF. No way.
The image quality of the ATi's was bad too. (Moving AA textures in GRID and shimmering in HL2) nVidia's GT200 series give a lot better IQ. AF and AA is much much better on the nVidia cards.
I'll never buy an ATi card again in my whole life. This was the second time I tried it, I owned a HD3870X2 too, but I sold them all after 3 days of gaming.
Conclusion: Dont believe reviewers... Test it yourself.