Do you have source for the 5850 for sale anywhere? It would matter how much you could buy one for, and also we should know how much you could sell yours for.
So, if you can find a 5850 for $40 used, then probably best to just buy that and crossfire with yours, because if you sell your 5850 for $40, you'd have to spend another $100 to get a 7870XT, but it probably won't give you $100 worth of additional performance compared to spending only $40 for crossfire 5850.
But really, I don't think you can find a 5850 for that cheap. So let's say you'd have to spend, ah, $70. Paying $70 more for 5850 crossfire is not worth it, because instead, you should sell your 5850 for $70, then spend another $70 to buy the 7870XT/Myst.
Note that you can also crossfire with a 5830 or 5870, so also consider whether those are available cheap in your area. I doubt it, almost all signs point toward selling your 5850 for the most you can get right now, before black Friday.