x2 gtx580 is worth?

Hopefully this will help you... Until recently, I was using 480s in SLI for years. When the 970 came out I finally went out and upgraded, and the difference was night and day. A single 970 blew away my 480s, handedly.

Yes I know the 580 is better, but not by a ton, and you're still limited by the 1.5GB of VRAM.

You're far better off just waiting until you can afford a proper upgrade. Another 580 will be better, sure, but you'll get way better value for your money if you wait a bit and get something better. You can also sell your existing 580 on ebay or something to help fund your future upgrade.