I have been using my 5870 for over a year and it rocks. However, I also use a 30 inch monitor (2560x1600) and I could always use more graphics horsepower. I am thinking of getting a 2nd 5870 (which should get me thru at least another year) or go with a 580 based card.
I play a variety of games - FPS's like COD Black OPs, MOH, Splinter Cell, and some RPGs (Oblivion, Darksiders, Devil May Cry)
I am able to play these at the full resolution, but I have to cut back on the AA and some of the higher end eye candy.
One issue you are likely having with your 5870 is your frame buffer. Assuming you have a 1GB card, at that resolution, 1GB cards sometimes run out of video ram when turning up the settings. Adding a 2nd 5870 will not solve this issue.
The end result is you will end up with higher average FPS with a 2nd 5870, but you'll sometimes have a higher minimum FPS with a single 580, or even a 480.
Often times it's the minimum FPS that make a game feel more playable, rather than the max. It's a tough call. If you are willing to turn down a few of the visuals when you play a game that runs into a buffer issue with the 1GB 5870's, you'll end up saving money and have higher performance most the time.
I would agree with karma831, if you see a good deal go for it(i hate waiting around)but go for the newer 580's or 69xx series that will come out soon.If their is a good deal get it and maybe down the road get a new mobo and x3 or x4 5870's.