Judging from the motherboard chipset I'd say 4x/8x AGP would be your choice, but to be sure that you have an AGP slot, pull the side cover off and look at the motherboard. If you have an agp slot it'll be brown (usually) located above the white pci slots. From what I've been able to findout the Dell 2400 didn't have an AGP, but has on board "AGP Graphics". That would explain PC Wizard seeing an AGP bus.
Have a look at the article below and you'll see what an AGP slot looks like.
http://www.techsupportforum.com/arti...pci-slots.html