I would like to know what resolution you are running. If you are running 1680*1050 then a 580 at 120 would be a decent buy, assuming you trust the owner to not have ran it into the ground. If you are at 1080p I think it is a little more of a decision. Currently the 580 does not have many issues with its 1.5gb vram unless you are modding skyrim or bf4. I just upgraded from a 580 to a 780ti. The 580 has the muscle to run bf4 with a mix of high and ultra but not the vram. It wasn't a huge issue but about one to three times a round it would hit it and plummet for a second or so. It was easily fixable by changing to a mix of medium and high.
I bring this up as it would appear that you are on a budget and would more than likely keep this card for a couple years. If that is the case and running 1080p I would be hunting for a sale on a 7870 or 660ti around the 150-170 price range.
Also, what are the rest of your system's specs?