Every motherboard has a PCI Slot...which sucks for graphics cards. You want to check and make sure (which I'm pretty sure you'll have it) that you have an AGP slot which is generally off centered of the PCI slots and is generally a different color. Usually brown, but now-a-days it depends on the motherboard manufacturer because I've noticed blue and red AGP slots too.
$100 won't get you much but no doubt it'll still get you a card that performs far better than your current onboard.
Still won't be anywhere near a 'gaming' computer though
EDIT: Also you should really look into more RAM. 512MB is basically the standard now and even that is getting kind of so-so as far as memory needs go.