I've considered Crossfiring another 4850, the problem being that I don't like the one I have now. It's got heat issues and hiccups far too often.
In that case, I'd say definitely the 5850 then. It'll still be a good card in your new machine, and in the meantime, while your CPU will be the weaker link, it's not terribly out of date. So any bottleneck will be high enough that you'll still be getting great performance in games.
And yes, you're correct about the memory issue. Crossfiring a 1GB card with a 512MB card is just going to be a waste; the 512MB card will be the retarded kid holding back the rest of the class.