I currently have 2 crossfired ATI HD4850 GPU's in my computer and I was thinking of upgrading to one ATI HD5850. I was wondering if there is a significant increase in performance for gaming and whether it was really worth the extra money to switch over to the 5000 series?
The performance at stock will be roughly equal but the HD5850 has a number of other advantages. It also overclocks VERY well, up to the performance of an HD5870 and even higher if you up the voltage. It uses less power than a single HD4850, it's dx11 compatible, supports 3 monitors and in comparison to your current setup it will have no crossfire scalability/compatibility issues. I'd say it's worth it if you can get a decent price selling your current cards.
I would say go for 5770 in crossfire
As these two 5770 will out perform 5850 and will give you performance on-par with $400 5870.
5850 is a good choice when you dont have a motherboard which supports crossfire.
Since you have a motherboard supporting CrossFire
I'd say you to go with 2x5770
And, you'll get
Shader Model 5.0
Upto three monitors support