You might be served quite well by spending on a new AMD am3 socket board with integrated graphics and a like a 880g or a 780g chipset. The low end processor is between $20 and $40 and the MB can be snagged for between $20 and $40. Most will come with IDE support letting you use your old drives. Ram will be quite cheap(10-20$), make sure you get ddr3 support.
For between $60 and 100 you will bet much faster overall performance as well as a pci-e slot that will let you upgrade to much cheaper and faster cards than you can get on AGP.
That is the route I took with my old system when I needed to upgrade it's video card.
You will spend at least $60 for any kind of decent AGP card.