If it was me I would wait for the 525 to see what is has to offer (Spec Wise, perhaps GDR3 ???) and pricing. 520 / 521 prices will most likely drop when 525 comes out has well.
But then again not sure how soon the 525 will hit the market after announcing has well.
I would wait if you can, unless you see a really good deal on 520 that you can't pass up. You could always buy the 525 later, if priced like the 520/521 are.