4870 and a 4850

umm, u already has that cards or not?
a 4850x2 are powerful enough i think...
What about the other spec?

no i dont have the 4850 x 2 i was thinking about gettign it but i herd the drivers and stuff were bad so im nto sure

I wouldn't recommend it even if it is possible. If you need more speed, I'd suggeest looking at the 5800 series as the next upgrade (or GTX3xx if/when it comes out).

^+1
yup, the new 5xxx series are more efficient, less heat, less power consumption and dx.11 ability, so if u want to upgrade then 5xxx series are better choice...  

Yeah, you don't want to add an HD4850x2 to an HD4870. The two options you should be considering are adding another HD4870 or Ebaying the one you have and going for an HD5850.