according to various web sites, your motherboard has suppourt for agp 1x, 2x, 3x, and 4x. practically all agp video cards these days are agp 4x/8x being compatable with both. so basically any apg video card should do fine, provided you have a powerfull enough power supply (if the video card requires specific amounts of power plugged into it)
you could also get a pci card, in which you could get any, but most pci video cards are more expensive than their agp counterparts (if the pci ones are even still made). so in short, you can get a better agp one than pci, especially if you are gaming.
now as for the card itself. i would recommend nVidia. that is my personal preference over ati. now to play call of duty, i had a pentium II computer with a geforce fx5200 (usually like $30-$50) play it fine, but you would probably want something better. so look for nvidia's 6 series cards. you can get a geforce 6200 for about $55. example: http://www.newegg.co...N82E16814170092
example fx5500: http://www.newegg.co...N82E16814145076
probably the best cards you can find for under $100 would be either a nvidia geforce 6600le, or a ati raedon 9600xt. i'm not very experienced with ati, but the 6600le should be able to play every video game out there today (not on highest quality settings) but it should play call of duty on its highest settings.
example 9600xt: http://www.newegg.co...N82E16814153012
example 6600le: http://www.newegg.co...N82E16814130262
hope this helps, post if anything unclear