I would get a GTX-670 since it is faster and used less power and less heat and the Gigabyte has two fans so it is quiet without haveing to put a different cooler on it.Plus they are just $389 and with the rebate it is $369,but the GTX-680 is comeing down in price arround $469.The GTX-670 is not much slower then a GTX-680 around 5% to 8% ,but the GTX-670 overclocks pretty good to make up the difference.There are a lot of GTX-580's on ebay looks like they are selling for around $250 ,you could probaly pick one up for aroung $200,but you are talking about a lot more power draw and heat and noise if you get a standard one.I read all the reviews and it just did'nt make since IMO to spend a extra $100 for no more performance then you get.So I ended up getting two ASUS GTX-670 Direct CU II and could'nt be more happy with my choice.What ever you get you should be happy with it ,but in some games the GTX-680 is faster then the GTX-590 and pretty close in others.
Newegg.com - GIGABYTE GV-N670WF2-2GD GeForce GTX 670 2GB 256-bit GDDR5 PCI Express 3.0 x16 HDCP Ready SLI Support Video Card