If budget permits, I would market your 4850 go with a single strong card.
Your 600w psu should be good for a card as strong as a GTX670 or a 7970.
But a GTX660ti or a 7850 would be a nice jump in capability.
I think your psu would be sufficient for 4850 crossfire but you could run into a microstuttering issue which could be annoying.
Read this: http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html