DOn't know if you've seen this page yet-
https://en.bitcoin.it/wiki/Mining_hardware_comparison#Popular_Mining_CardsI've got two 5830's running on two different rigs. They were both machines I already had so the setup costs weren't too high. I'm not getting the speeds shown on this page, maybe I'm not tweaking enough. But one does around 230 and the other does around 250. The 5830 cards aren't that expensive. There was a guy on Ebay selling around 20 of them at a low price, I only bought one that time but maybe should've bought more. I think running two cards on one rig might be a hassle, but not sure as I haven't tried it. I have lately been thinking about adding a third rig since the price of btc has gone through the roof.
Other than making sure that your rig has enough fans for cooling I don't think the computer itself, the motherboard and processor, need to be fast; it's all about the card. So I think as long as you have a PCI-e slot in the machine you're good. I might just get my next card first and then get the machine- before I already had the machines and got the card. I don't know why but the best cards for mining don't seem to be the newest ones, so you end up getting them on Ebay.
If this helps send some bitcoins!
1Der277TZjY3UHS1j9FV7iRMRKc4N985mS