Maybe you could consider a 5830 if the 5850 is too expensive. HIS makes a very, very good card here: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I've never had a single problem with any of my HIS products, and the 5830 will give a nice performance boost over what you have now.
OK lets put it this way.
If you can afford a 5850,then sell your 4870 and get it.Otherwise get another 4870 and CrossFire,both will give you roughly the same performance(2x4870 would be about 10%-15% faster)but 2 4870's will produce more heat,more noise and have more power consumption
For the money, a second 4870 would probably be the way to go, but you'll want something bigger than a 550-watt PSU if that's what you're planning on. The only problem with this, that nobody seems to have paid attention to, is the fact that your motherboard only supports a single graphics card.
...for set-up see signature
------------------------------ ASUS M3A78-CM,X4 940 BE@3.6GHz,4GB G.Skill@800mhz,HIS 4870 1GB GDDR5,OCZ Fatality 550watt, 640GB WD Black, Win 7
I'm still sticking to my theory that you should get a 5850 if you can get the money for one, and if not, then go with a 5830.
HIS 5830 HIS 5850
No, I'm not an HIS fanboy. Those are just IMO the best cards in their price ranges, and they both happen to be from HIS.