Personally if you really can't wait till ATI release there new cards I would get the 580 just simply for the reason that it is quiet (some say that it is the quietest card on the market at the minute compared to the 480, 5870 and a 285 which was overclocked iirc etc.) and runs very cool and of course is the fastest single GPU card on the market at the minute.
However personally IMO I don't think it is worth the price at the minute for me to even consider buying it, it would have to be around £370, but that is up to you if you want to spend that much considering it is really just a 485 (what fermi should have been).
Also it depends on what resolution your using, if you are going to be playing games at 1920x1080 then the 580 would probably be the best choice for single GPU and if you wouldn't mind going SLI/crossfire then probably 2x6870, 2x460 or 2x470 (although the 470 run very hot and noisy, but they are at a very good price at the minute to make that up in some ways

)
Also you need to take into consideration, how many monitors you might want to run or are running at the minute as 1 nvidia GPU (580) can only run 2 monitors max iirc (you would need to go SLI in order to run 3 or more) and with ATIs cards you can run up to 6 iirc (with the 6xxx series that is).