Sorry, just trying to help. There are freeware / shareware programs that can be used, though I've never used the AGP settings on the ones I've used. PowerStrip is a great video tool. I use it for OCing, but it has more features than I know how to use. I'll have to check if it has a way to set AGP speeds. If you've already tried this or it doesn't work for your OS, then you may have better luck in the video forums.
EDIT: PowerStrip can be used to change AGP speeds. Works on my system (WinXP), and since the prog is OS-independant it should work on yours. Hope this helps.