It will work but not as fast as a regular AGP slot. I'd steer clear if I were you. It's basically a regular PCI slot (not PCI Express) that you can plug an AGP card into. If you want a true AGP slot you'll have to get a motherboard with a slightly older chipset. Unless you plan on upgrading to PCI Express soon its pretty worthless.
On the other hand if you're planning to upgrade your video card soon to one based on PCI express, it may be worth getting as a stopgap measure, then you can upgrade in stages.