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
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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.