Some setups do not work too well with certain settings like Fast writes, AGP 4x, or 1 wait state AGP read/write.(You may want to print this out or write it down)
Reboot your computer, and just as soon as you see the initial screen (after the screen showing 3d prophet info), hold in Delete. Hold it in until a blue screen pops up. This is your bios.
Before you change any settings, write down the names of them and the setting they were on before you changed them. That way if you screw something up, you can do 2 things. 1) Not bitch at me
and 2) Change the setting back to the original one
Go to Advanced Chipset controls and scroll down until you see Fast Writes. If it is not disabled already, do this.
Find AGP 4x mode. Disable this as well.
Down at the bottom of the page should be two items with "AGP 1 WS" in them. Set these to disabled if they're not already as well.
There is an AGP Aperture setting as well (I think it is also in Advanced chipset). Play around with this setting, because it can produce the result that fixes your problem.
Hopefully this will work.
If the above settings were all already disabled and none of the AGP aperture settings fixed it, then you could try resetting the bios to fail-safe defaults. If that doesn't work, try optimal defaults.
Usually problems like these involve the bios, and it is easy to send it down FUBAR street. If you manage to get your computer to stop even powering on or booting, calm down. You CAN reset your bios.
Unplug your computer, and consult the manual for the motherboard. It has all the instructions you need to follow to be able to reset your bios.
Hopefully you will not need to go that far though.