Hamiac, you missed his point. He didn't say the 4850 was as fast as the 5770. He said that because the 5770 is slower then the 4870, what your suggesting is worse then telling him to spend $$$ to move up to the 4870. Depending on his resolution he might not even notice the speed. If you have a 4850 already, and game at a low enough resolution, it doesn't make a lot of sense to spend a lot of money to get more power which you can't use.
Moving to the 5770 only really makes sense if there is some game he wants to play that the 4850 just can't handle, is DX11 only, or if he wants the features found in the 5 series like Eyefinity. It also makes sense if he can get the 5770 cheap enough, and sell his 4850 for lots of money. If he can get the 5770 for $150, and sell the 4850 for $100, then the 5770 only cost him $50.