Here are the agp card benchmarks:http://www.tomshardware.com/2005/07/05/vga_charts_vii/page4.html
As you can see a lot of the cards will out perform the 5900.
Biggest problem you will find with AGP is trying to find they seem to sell out quickly.
The agp x800gto was a great card but sold out very quickly.
All depends on your budget on waht card to buy but you can get an idea of what to expect from the graphs