In some cases, it is slower and less stable than agpgart. For certain supported motherboard chipsets, nVIDIA recommends the NvAGP driver instead (see Appendix F of nVIDIA's README for your driver, available from their Unix driver page).
Agpgart is usually preferable (except where noted above), so it is given priority in the system. Even if you have "NvAGP" "1" in xorg.conf, agpgart will be loaded. To use NvAGP, you must either completely remove agpgart from the kernel or boot with the "agp=off" option.
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