I play Call of Duty 4 alot, and need a better graphics card. Any help?

i have a http://h10025.www1.hp.com/ewfrf/wc/doent?product=93330&lc=en&lang=en&cc=us&dlc=en&docname=c00009514 computer right now im running a Radeon 9800 PRO which is minimum for cod4, everything else besides the Graphics Card (radeon) is stock, i know its a AGP 4x, what is a good compatable graphics card for me to play on COD4, also what else might help me with fps etc.