I would say it wouldn't be a problem. There were a few driver bugs specifically with the 7850 when it launched but they should have been sorted out by now.
The upgrade from a 5850 to a 7850 is decent but I wouldn't really say its worth it right now. depends on what you want tho. The 7870 would be a bigger jump but a lot more expensive. Maybe wait for prices to fall.
That is one of the reasons I am still a little hesitant. Although moving forward, do you think the motherboard/CPU will be a limiting factor since the mobo is PCIe 2.0?
I originally bought the system to add a 2nd 5850 to. However by the time I could have used one, they have all but disappeared and are as expensive as better cards. If not more expensive.