ur hunch on the high setting is not really off the mark -
" ... AGP Speed
(1x, 2x, 4x, 8x): This setting determines the data transfer rate (speed) of the AGP Bus - the pipeline along which video information flows. As logic suggests, the higher the speed setting, the higher the potential performance of your graphics card. However what is counter-intuitive is the actual performance difference between the various AGP modes. AGP 8x is not twice as fast as AGP 4x, which is not twice as fast as AGP 2x - and so on. The reason for this is that the higher modes provide more bandwidth - that is, the size of the pipeline effectively gets bigger. But if at AGP 4x the pipe is already bigger than the size of the information flowing through it, increasing the pipeline by going to AGP 8x will clearly not make a large difference to speed. Importantly, lowering the AGP speed (e.g. from 8x to 4x) can help improve stability for some systems (especially overclocked ones), and can also resolve some graphics problems. Generally I recommend setting this to the highest available speed in the BIOS, as long as your graphics card also supports that AGP speed (See ATI Control Panel/ATI Control Center sections). For most systems the performance difference between AGP 4x and AGP 8x is at most around 5-10%.AGP Aperture Size
: This setting determines the allocation of physical RAM for use by your graphics card, should it become necessary. A better description can be found in this AGP Aperture Size FAQ. To decide how big it should be keep in mind the following: (1) keep it above 32MB, as an Aperture Size below 32MB will disable AGP texturing - reducing your performance greatly in games; (2) the greater the amount of Video RAM on your graphics card, the smaller this setting should be; (3) Values between 64MB and 256MB show no real performance difference; and (4) Using larger values can result in more crashes such as General Protection Faults and potential texture corruption. So with all of these in mind, I recommend an Aperture Size of 128MB for most modern graphics cards. If you have an older 32MB graphics card, set the Aperture to 256MB. If you experience a large number of crashes or texture glitches, try lowering the Aperture to 64MB regardless of your Video RAM amount.
the whole reading is at http://www.tweakguides.com/ATICAT_2.html