First: What is the rest of your spec?
Toms has just released an article on the 4770 in Crossfire which includes benchmarks for both single and dual cards (obviously dual;)). Read it and be amazed.
As a single card upgrde it sounds like a good idea and would give your games a good boost without being too expensive.
As an idea, look here:http://www.tomshardware.co.uk/radeon-geforce-gtx,review-31554-6.html
The 4770 is not listed because it was released after this chart was compiled but slots in just under the 4850.