voilà ce que j'ai trouvé avec google (en anglais) :
AGP Driving Value
Common Options : 00 to FF (Hex numbers), 00h to FFh
Quick Review
This BIOS feature will only be activated if you set the AGP Driving Control BIOS feature to Manual. It determines the overall drive strength of the AGP bus.
The drive strength is represented by Hex values from 00 to FF (0 to 255 in decimal). The higher the drive strength, the greater the compensation for the motherboard's impedance on the AGP bus.
In conjunction with AGP Drive Strength and AGP Drive Strength P Ctrl, this function is used to bypass AGP dynamic compensation in cases where the auto-compensation circuitry cannot provide adequate compensation. If you are using an AGP card built around the NVIDIA GeForce 2 line of GPUs, then it is recommended that you put AGP Driving Control into Manual mode and set AGP Driving Value to EA (234). For other cards, please check with the manufacturer if your card requires the AGP driving strength to be manually set.
Incidentally, increasing the AGP drive strength will not improve the performance of the AGP bus. It is not a performance enhancing feature so you shouldn't increase the AGP drive strength unless you need to.