The 3dfx Voodoo series of cards are not TRUE AGP cards like most of the Nvidia cards. So adjusting any AGP driving strength or value will have little if any effect. Here's some info from a site I can't recall tho on that portion of the BIOS.
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AGP Driving Value
Options : 00 to FF (Hex numbers)
This option is slaved to the AGP Driving Control BIOS function. If you set the AGP
Driving Control to Auto, then the value you set here won't have any effect. In order
for this BIOS option to work, you need to set the AGP Driving Control to Manual.
The AGP Driving Value determines the signal strength of the AGP bus. The higher
the value, the stronger the signal. The range of Hex values (00 to FF) translates into
0 to 255 in decimal values. By default, the AGP Driving Value is set to DA (218) but
if you are using an AGP card based on the NVIDIA GeForce2 line of GPUs, then it's
recommended that you set the AGP Driving Value to the higher value of EA (234).
Due to the nature of this BIOS option, it's possible to use it as an aid in overclocking
the AGP bus. The AGP bus is sensitive to overclocking, especially in AGP4X mode and
with sidebanding enabled. As such, a higher AGP Driving Value may be just what you
need to overclock the AGP higher than normally possible. By raising the signal
strength of the AGP bus, you can improve its stability at overclocked speeds.
But be very, very circumspect when you increase the AGP Driving Value on an
overclocked AGP bus as your AGP card may be irreversibly damaged in the
process!
BTW, contrary to some reports, increasing the AGP Driving Value won't improve the
performance of the AGP bus. It is not a performance enhancing option so you
shouldn't increase the value unless you need to.
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Hope this helps.