Read more here: http://en.wikipedia.org/wiki/R520#Variants
I also ordered a (cheap) R600 (or R700? don't remember...) based card to experiment with or put on the shelf for a few months; people seem to be working quite hard on ironing the bugs out, and those will probably be well supported in the future.
I ordered my R580+ based card from a place called compuvest. It was my first order with them so I can't say whether or not they're worth recommending. They seem to have a lot of generation-ago equipment. Another place to try would be geeks.com, which is where I got the R600 based card.
I'm not sure how well this will work out, but that's what I'm trying...