It also depends on your CPU.
The AMD X2-4800+ can not take advantage of anything above an HD4770. There is a CPU chart that shows relative CPU performance.http://www.cpubenchmark.net/index.php
As for which card, provided your CPU can support it, simply get the one with the most performance for the money. Tomshardware has lots of benchmarks.
For your money, if you can get it the HD4770 is an excellent card. An HD4850 is a great choice too as mentioned.