The 5850 outperforms the GTX 285 pretty easily and comes in at a much nicer price.
The 5870 really spanks the GTX 285 for about the same or maybe a bit more money than the GTX 285.
The ATI 5xxx series provides you with eyefinity, DX11, and good performance for the money.
If you can't wait to see what Fermi will bring, then there is no reason not to get the 5xxx series in your price range.
ATI does not have poor drivers, they are no worse or better than nVidia's drivers. I only mention this because there is a lot of bias and misinformation stating otherwise out there, it simply is false.
If you absolutely MUST have a nVidia card and you must have it now even though you will be missing out on features and a good deal of performance, then I recommend that you look into the GTX 275 which can be had for far less than the GTX 285 and provides similar performance.
Another thing to keep in mind is that the 5850 or 5870 will draw less power than the GTX 285 or GTX 275, so if you are worried about your PSU...
Hope that helps.