I find it will hang at times. I've got dual 6850s in crossfire but they still struggle with 1080p sometimes. I'll overhaul the rig next year probably. Problem is it needs new everything. Even my case is starting to fall apart, and the DVD drive is IDE.
Doesn't the 6850 have 1GB of RAM? Crossfire and SLI provide both cards with the same data, which means you still have 1GB of RAM. I would guess that's your biggest issue. I play it at 1080p with 2GB, and it was
fairly good. I had a few hitching problems in the beginning, but it seemed to get better. I don't know if there was a patch or anything that helped mediate or if a setting got turned off randomly. I checked my settings one time, and I noticed that a few of my settings were turned down, but I didn't turn them down. :\
EDIT:
WoW for some time could really be a bitch depending on which options were selected. I remember that shadows, for instance, did have a very dramatic effect and would slow down the game a lot. That's a few years ago, though, so it probably wouldn't matter with today's hardware anyway.
It just comes down to light-modeled, model-dependent shadows requiring significantly more processing than other solutions. For example, the easiest and laziest thing is just to throw a black circle under the player like the sun is always above them. Then there's blob shadows that take into account the light source's position in regard to the model.
Also, keep in mind that if you're in a big city, you have to perform those calculations for
every player model that's currently running around. WoW also used the shadows for things like trees and such, which fortunately, are far more static.
EDIT 2:
WoW - through first or second expansion (I don't remember which one). I was pretty angry this one, I've played vanilla (or TBC) back then on highest possible configuration and then when the expansion came out, I had put everything down to medium or lower in order to not get a lagfest in a raid.
I wonder if it was the significant increase in particle effects? I remember that my 8800GTX had some bad problems with some effects. For example, if I stood in Hellfire Peninsula when the fire erupted from the ground, I would watch my framerate drop 30-50%. However, I'm not sure if that was the video card (I don't think it ever worked right), poor drivers (I used Vista... that says a lot), or poor tweaking on Blizzard's part.
I'd probably be more willing to blame the video card over Blizzard for that.