In single card configs, the 58xx seems about 10-15% faster than the equiv 68xx. In crossfire, the 68xx scale better making them almost completely even with their 58xx counterparts.
With that in mind, you can probably use this comparison (which shows the 5850 almost the same as a 5970):http://www.anandtech.com/bench/Product/165?vs=183
As far as those two cards, I think it is a toss up (I'd probably get the 5970 just because it is a single card and maybe you can SLI them down the road if you want).
For $500 the AMD solution I'd get would be the 6870 CF.