It is a feature supported by some motherboards, chipsets, and video cards that gives a very slight increase in performance by allowing the AGP bus to transfer data more efficiently.
It is known to cause instabuility in alot of cases however, so most people disable it because it isn't worth the meagre performance boost.
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Radeon 9500 w/256 bit memory bus @ 367/310
AMD AthlonXP 2000+
3dMark03: 3439