The HD 5870 is from +10%, all the whey up to +155% better than GTX 285 in 17 different game's. However under Crossfire it might have serious problems. On general crossfire scales better than SLI (A test done by benchmark.pl) but as we all know this is a new GPU and the drivers aren't really out there yet. So we are yet to see, if HD 5870 does as good as it's predecessors did in that matter. So HD 5870 Crossfire? A bit of a gamble ;] But, there are more things to consider. the eyefinity technology will let you connect up to 6 monitors (in crossfire mode). You get the directx 11 with a bunch of ridiculously good features. Such as Direct Compute 11, that might not tell you anything, but in short, it’s a standard for what ATI and Nvidia are trying to do with their “ATI Stream” and “CUDA” respectively. It’s important to notice that (unlike with DirectX 10) you get a few games - almost at lunch – that can use DirectX 11, with Battle Forge, Dirt 2, And Stalker: Call of Pripiat on top. But one very important question you asked, haven’t yet been answered and that is if ATI has something similar to PhysX. Unfortunately they don’t, and by the time they will, Programmers will most definitely have started using Direct Compute 11. Since any DirectX 11 GPU is compatible with it. And you can use it for any calculations (Physics, A.I., Video Encoding/Decoding etc.)
So it’s like this:
GTX 285 SLI is most likely (but not indefinitely) slower than HD 5870 Crossfire
HD 5870 Has allot of new technologies with DirectX 11 sitting firmly on top
GTX 285 Has PhysX which as you know is in most games an important feature
I personally, would go for HD 5870. When the GT 300 will hit the market. GTX 285 will undoubtedly become old. It’s SLI will still have enough power to run new games flawlessly. But it wont be able to. Because it’ll lack DirectX 11 compatibility. HD 5870 on the other hand might turn up to be a great GPU given enough time for ATI to optimize it’s drivers.