No, you are talking about two different things. AGP 8X is the newest AGP specification for the next generation of video cards, like the Radeon 9700. The bandwith that is listed in the specs is the available memory bandwith on your card. You will get the full bandwith when using your card in an AGP 4X slot. 🙂
Edit: D'oh, Dulanic beat me to it. 🙂