I have a 580 in checkout basket now,but as i currently own a 480 with a custom cooler,my head is telling me i dont really need it.
Though my thinking is, the 480s price is going to get really low soon when trying to sell second hand, especially with the 570 and 6970 cards when released,so maybe its best to sell the 480 now while i can get half decent money for it.
Ah decisions
Dont do it!!!
Check this;
option 1 - sell you 480 for say £290 and buy a 580 for £400. Cost £110 - extra 10fps - wil you notice this - I couldn't
option 2 - buy another 480 in a few months for £250 ish - nearly double your frame rates! (eg HAWX @ 2560x1600 up from 75 to 141fps)
The 480 is going to be fine - wait for the prices to fall and get another for SLI in a few months and you'll be set for a long while