The best way to get a real idea is hardware compare. Unfortunately I just looked and they do not have the stats for crossfired 5770s (they have crossfire stats for many other cards) which makes it harder to say. I did notice your rig wasn't that far off mine on the heaven benchmark though. I was surprised by that to be honest. I have a 5850 and this gtx580 is an improvement over it, but not a drastic one with the exception of maybe batman arkham city, and bf3 with AA on. Where you really can tell how much horsepower the gtx580 has. I still consider them both good cards.
I'm rambling now, the truth of the matter is I was hoping hardware compare would answer a lot of your questions and it still might if you just jump between comparing various cards you have a good idea about. Performance Comparison Benchmarks @ Hardware Compare
As you can see the gtx580 at least in the way of memory bandwidth is a good marker. I still consider it a good buy, but as you can see the 7970 is noticeably higher.
(and I got my gtx580 for 100 bucks plus shipping from ebay)