He could try to put a 7800 in as that was the best agp card that he could get but for just a little bit of money he can get a system way faster:
(you can even use the same case!)
So for a bit over $515 just do this:
http://www.newegg.com/Product/Produc...82E16819115222
http://www.newegg.com/Product/Produc...82E16813188063
http://www.newegg.com/Product/Produc...82E16813188063
http://www.newegg.com/Product/Produc...82E16820104155
http://www.newegg.com/Product/Produc...82E16814130564

You should be able to get great fps in all modern games with this. This setup is a major improvement over your current. Just reuse all your other parts. All you need are a new CPU, M/B, RAM and GPU. However you might need a new psu if it is not 500w but even a system that old should have about that.

Also do not try to give me some bs about oh it is too hard. If you can put a lego set together you can build a computer. Lastly it is important to note that the 460 I selected ends in -AR this is so that you can get the lifetime warranty if you register it in 30 days. This will make it so you have great protection on most of your new stuff. The ram for example also comes with a lifetime warranty. The motherboard has a 2 year and the cpu is 3 years.