I have got a EVGA GTX 480 and I am starting to come across games that it struggles to display on maximum detail I am kind of confused on which option to choose from so I thought I would seek council with you guys

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I am thinking of buying a USED EVGA GTX 480 and running it in SLI with my EVGA GTX 480 but I am not sure on this, How much would I be looking at spending on a stock (not SC) EVGA GTX 480?
My other option is to sell my 480 then save a whole lot and buy two 580's and run them in SLI, but I don't know how keen I am on doing this.. I would "Hate" to finally get the cash buy them then a week or so later have the 600 Series jump out on me.
I would have thought of getting the 590 but in all the benchmarks I have seen SLI 580's almost always wins and there is no way I am paying $2000 AUD to buy 590's in SLI lol.
So what should I do? would buying a used 480 be a waste of money better saved up towards dual 580's or are they not even worth it because soon a 680 will be on the shelves or something lol.
Thanks.