I would recommend against the 4870, because it won't have the performance of the 4850 x2 or the 2 4850's you currently have.
As for the Processor, or the Video Card,
I'd say the processor. I'm not sure the 4850x2 would increase performance much, if at all.
As for the Processor, or the Video Card,
I'd say the processor. I'm not sure the 4850x2 would increase performance much, if at all.