Depends on what you are doing. Some games are better on one brand or the other. Almost any card is better than onboard. and memory is like money, more is better. AGP 4/8 is at the end of life, PCI is the future, and should be considered for future upgrades.
You can get a Radon 9800/256 for about $180 and it will work quite well. I was able to Put up some real good marks with the one in this system. But then I had to pay through the nose for it 18 months ago.
Benchmark