what kind of performance gain have you gotten by adding a second 5850?
Not as much as in any other game I tried.
At 1920x1200, without any mods and no config file:
All settings on High (no AA but the edge_AA you get at High automatically):
Single card: 51-ish
Crossfire: 57-ish
So not much more than a 10% increase there. It's worth turning the settings up a bit more though:
All settings on very high (same AA as above):
Single card: 30-ish
Crossfire 37-ish
So 25% increase. More importantly, this feels smoother. I did find when motion blur is very high, the game occasionally freezes when I turn rapidly - that happens whether single or crossfire. So I drop it to Medium.
I can't help feeling I should be getting more than 25% though...
Although it does let me pile on the mods - natural mod, that big texture pack, HD foliage, and only lose 1-2 fps. Whereas my experience of trying them before getting the second card wasn't fun.
I haven't done much playing with the mods yet since getting the new card, so don't know how smooth they'll be in play or whether I'll have to turn some settings down from Very High yet.