I have a 9400gt in my media machine and my bro uses a 6770 in his gaming machine. The 9400 is a great card for light graphics work in a low power computer, like a HTPC. It does not have the power to do heavy video games. It costs around $40.
The 6770 is a fully capable video game GPU. It would be considered low to medium end but it is very powerful. It is around $120.
In terms of video game FPS the 6770 would be about 4 times as fast, so if the 9400gt can get you 15 FPS I would think the 6770 could get you around 60.
I did not run any newer benches on the 9400 but i can get around 3,500 in 3dmark 06 with a core 2 duo. The 6770 will get around 12,000.
If you want to play games and need a powerful GPU the 6770 is the way to go. If you want a quite lower energy gpu for basic computer needs, like movies, and MS office, and dual display you can use the 9400gt.