Three choices for you.
1) Buy it and use the video card in existing system, and sell of board (may get $30-40 for it).
2) Buy it and use both... and buy a socket 754 CPU. Will get a performance boost over existing system if the true clock speed is same or greater than your existing setup. IMO not worth it, just use board to make your video card purchase cheaper - see #1.
3) Buy a PCI-E video card, CPU and motherboard. Will cost a motherboard more but benefit is that for the same money you can get a faster PCI-E video card, example being 7800GS AGP versus 7800GT PCI-E for same cost, the 7800GT has more pipelines. Other thing is that you wouldn't be putting more money into AGP. It's not that AGP isn't good enough, it's because manufacturers don't want to put the "better" GPUs onto AGP boards anymore.