If you're going to buy new stuff just go ahead and get a new graphics card while you're at it. Your AGP card may work, but it's not anything great and they don't make new motherboards that support AGP.
Something like this: http://www.newegg.com/Product/Produc...82E16814130098
You obviously don't need anything amazing, and that is cheap and will work with a new mobo with a PCI-e slot.
Seriously, the only mobo I could find with an AGP slot was a Pentium 4 LGA775 socket mobo. That is long since outdated, so nowhere near "top spec." If you're going to upgrade, just go all the way. You will need at least a motherboard, processor, RAM, and a video card (unless the motherboard has onboard video).