Well, maybe...
I would have said no 6 months ago, but seeing the hybrid Xfire on the 780g makes me unsure. You usually need the cores to be the same (that's why 3850 and 3870 work together, the 3850 is basically an underclocked 3870), but I guess we will have to see once they come out. It's possible, but I wouldn't bank on it. Wait for the 48xx if you can and will be able to afford them ($200-300) or grab a 3870 and cross your fingers.