Thats fine, as I stated both choices would be awesome, and I really just can't see how if you had a 4870x2 that it was horrible or if you had 2x 4890's, unless you are running Vantage all day long and feeling very happy about a artificially high CPU score.
the horribleness can be attributed to a few things.
Temps+noise, it was one or the other, 60-65 degree loads and two hairdryers in the room, or 85+ degree loads at an acceptable level.
Build quality, As i understand it release 4870's were all identical, and boy did the quality shock me. both cards buzzed, somewhat violently, and both had a cheap looking PCB, on which the white lines were printed crooked.
As for gaming, i started with one and added another, to notice largely that only max FPS was increasong and MIN was EXACTLY the same as a single 4870. Some games favour CF unnaturally, just like SLi, and those were pretty good.
My immediate move from 2x4870 512 in CF was a single GTX260 65nm, and my experience was worlds better for it. higher min FPS and lower max, so less variation, and MIN is really what matter to me, i coulnt care about anything over 120 fps, but below 60 shits me.
the GTX295 proved similar to CF, where i could often find myself on single GPU performance, the saving grace here is 1xGT200 with 240 Sp's, in my experiences, is a decent amount more to fall on that a single 48XX chip.
Albeit the 4890 does par a GTX275 on the whole, so really they should perform about the same, so performance aside, my other points are valid i believe. space, heat, noise, build quality. and something imonna throw in for the first time right meow, is drivers. IMO nvidian drivers take a steamy dump on ATi's, but each to their own.
