EDIT: I guess the 580 is okay, it's taking about 4 Times as long to run the Moo GPU Wu's but it's getting about 4 times the Credit too so I got Excited about nothing on the 580 ...
... I need a spare Cheap yet fairly productive NVIDIA Card so I'm thinking about a GTX 660, can get one of those for under $200, a 670 cost over $300 & I don't want to spend that much on a card ...