The 5870 is essentially the 6870, as you can see, the 4870X2 are pretty much matched up so if you went 4890 CF it'd be pretty good, but power consumption and it creates WAYYYY to much heat. I think you'd better go with a new 6870.
I know its about $20 higher than your budget, but i think its good enough. I'm also assuming you dont need a case, and you can use your old one. From what i have read you can OC it to 3.6 ghz without the need of an after market cooler. I also think 6850 will be better coz it will keep temps and power usage down.
-The combo has an additional HDD, which you could probably sell later, along with the 4890 and get some cash back.
-i'm trying to keep it around the OP's budget
-Your suggestion a few posts above doesn't include a PSU
-If you have a 550W from a good company(Antec, XFX , Corshair, etc) is better than having a 750W from an unknown company. Besides AMD's GPUs consume less power and you will be fine inside 550W
-I suggest you take the i3-2100 as its wayyy better than the x4 955
-Its up to you if you want to increase the budget, but its the best i can give you with in $500