4770 or 4850

The HD4850 is better.

+1^ HD 4850 is a better choice

4850 of course...

I would say get the HD 5750 ^.^

In terms of performance:

4770 < 4850, 4850 ~ 5750

If you are still using Win XP then avoid the HD 5xxx for now since there may be some performance issues. I'll find out for myself once I install my HD 5850 and test it Win XP and Vista.

Both 4770 and 5750 consumes less power than the 4850 in case that info is important to you.