I've been running an Asus 9800GX2 TOP for awhile now and on impulse, opted for dual Sapphire 4870's over a GTX280 for my latest upgrade on the weekend.
So far I've only installed one card because I still need a new case to fit two cards in neatly, but I can say I see no slowdown in performance compared to the GX2. On the other hand, when the 4870 spools up it's fan, which is does quite often, it's much louder than the GX2. I still trust nVidia with drivers more than I do ATI, so I'm not a complete convert yet. But for the dollars spent, I can't complain much with how just one of these cards performs once you set it up. Damn things are longer than the GX2, too, so if you're short for space that could be a problem.
Might be worth you checking out depending on the pricing in your area. Could be a case of nVidia no longer being the default choice.