The HD4770 runs at 90% the performance of the HD4850. It's quieter and consumes considerably less power though as it uses the new 40nm process (it's the only desktop card that does).
I was disappointed in some recent benchmarks though. Have a look at the latest Tomshardware. Keep in mind that your CPU can't take advantage of the top (of the mid-range) graphics cards like the GTX275.
It varies by game. I thing you could still take advantage of a step above the HD4850 for many games with that CPU (see the ranking chart).
There are many games that would look great with the HD4770 though and quietness and heat are important factors too (and cost).http://www.tomshardware.com/reviews/best-graphics-card,2362.html