To avoid system instability caused by a conflict between the speculative cache that the AMD XP processor is using and the memory use of AGPGART, AMD Athlon XP systems should include
Option "NvAGP" "1"
in the device section for their graphics card in XF86Config-4. This will allow the nvidia AGP driver to be used in place of AGPGART (assuming that AGPGART is being loaded as a module).
My system used to lock up occasionally while doing intense opengl grapics (especially in FlightGear or in some screen savers). I made this change yesterday and have had it running with either FlightGear or very complex screen savers since that change, and it has been completely stable.
The following link is to an article that explains in detail what the issue is.