I have talked to the store where i bought my GTX570 1280MB OC and I can switch it to a GTX580 1536 OC for about 106 EUR (24% more). I have checked the BF3 tests on this site and the diffrence seems to be small? not even 20%?
I'm going to put it this way for you, the 5-7 fps differences are when it's at low fps to start with, around mid 20s. At that point, 5-7 fps is 25% smoother gameplay, furthermore it makes a much bigger difference visually. It's once you get past 60 fps that the differences don't make much of a difference because it's all smooth at that point. I think it's well worth it to get the 580 if you look at it that way. At the same time though, it depends on what settings you play on, esp. what resolution you play at.