I just purchased and installed the Gigabyte GTX760 GDDR5-4GB 2xDVI/HDMI/DP OC and I am loving it! It just so happens that I have an i7 3770 and it runs awesome! The psu requirement for this card is 500w, you only need 700w when running sli, so your 600w will be fine. I am running mine with a Corsair 600w PSU, so I know it's enough. You'll only need the 4gb card if you plan on modding your games with a lot of heavy texture mods, like Skyrim (which is what I do), otherwise 2gb should be fine. Now, I don't know how important the cooling is for you, but living in Florida, it's
very important to me and I chose Gigabyte because their version has 3 BIG fans and your 4gb card had some really nice BIG fans. Also, you may want to get the 4gb for future games and then you may not have to upgrade for a while. I think it's worth the extra money right now; no regrets later. It's really up to what and how you're going to play. At any rate, the GTX 760 is a really nice card and you have the power, all you have to do is decide which one. Hopefully the info I gave can help you make that decision. Good Luck!!!
By the way, I upgraded from a GTX 660 also
Reference for PSU Requirements:
http://www.realhardtechx.com/index_archivos/Page362.htm