Graphics Card Upgrade Advice Wanted

Hello,

I bought a nice machine two years ago that came with two NVIDIA GEforce GTX 280 cards in it. In the last three months, these cards seem to have degenerated (no doubt this was brought on by an overheating problem caused by a dust cap forming over the fins of one of the major heat sinks inside the case, which I didn't notice until the system started acting unstable).

I plan to check my power supply this afternoon when a friend brings his multimeter over, and if it checks out, I'm going to assume the cards are fried.

What I want to know is what a good replacement for the cards I have would be. Unfortunately I can't seem to access the online specs for my machine anymore (I should have printed it out, I know...) All I can find is this: WPC/i7-940/6GB-DDR3/2xGTX280/SLI/SAS/VHP

The above is from an e-mail on my order from tigerdirect. I can decipher a lot of it (WPC = Widow PC), but not all.

I'm looking at the NVIDIA GEforce GTX 580, 570, and 560ti cards.

I'd like to know if I *have* to buy two new cards or if I can just go with one new one replacing both of the old (does my system REQUIRE two cards?) Does dual-carding really enhance performance that much? I'm not averse to paying for two cards, but I wonder if one lone card will run cooler than two.

Will all three of the card types above run dual, or only some of them? Are they good choices?

Any advice would be good. Thanks!