I got a Powercolor 5850 for $160 almost a year ago, I was planning on upgrading with Crossfire, but I am in the same boat since 5800 series cards are very difficult to find. I don't know if it was just the fact they were always in short supply or because of their raw performance in Bitcoin, but you will have a difficult time finding one and even if you do I doubt it'd be worth it. I see them on Craigslist and Ebay sometimes for as much as I paid for one new.
And no you can't use a 6800 or 6900 series with a 5800 in Crossifre, even if there was a work-around there are already enough application issues with Crossfire/SLI.
If I can get another 5850, can I use any brand or do I have to use the same bran Im using now?
Brands don't make a difference that I know of, the only downside of having a higher clocked 5850 or a 5870 is that the Crossfire pairs up the lowest denominator unless its like a dual GPU card like a 5970 then its pretty much three 5870s.
I haven't run a Crossfire set up before, so I may not be right, I was just sharing my woes about the lack of 5800s making my ability to ever Crossfire before upgrading very slim.