SLI or getting 1 better card when GPU gets too old

Depends on your PSU - if it will support SLi - 2x560Tis are a good combo - way more potent than a GTX580. If you have to buy a new PSU + GPU a single card would make more sense. Most new cards are more powerful/efficient so you could most likely reuse your current PSU.
-Bruce

Cooler Master GX 700W good enough?

700W is the min. Cooler Master is not one of the "stellar" PSU manufacturers but they do make decent products.
http://www.tomshardware.com/forum/321076-28-650w
-Bruce

Yeah you don't want that PSU running one card let alone two cards. Or even a new more powerfull card. That PSU is on tier 4 and tier 5 is the worst. That PSU if it was a 450w then is would be fine.

You will see when I link you to it. I don't think you can throw up the old GPU. I would do the SLI first. Look at how many people are still using the GTX 260's. So they will still be good for a while.

If you get a new PSU look for the brands Corsair, Antec, Seasonic, and XFX. That would be your best bet. And I do wish you good luck on you problem.

http://www.eggxpert.com/forums/thread/323050.aspx