This BIOS feature controls the AGP bus' Fast Write capability. Fast Write is a feature which accelerates memory write transactions from the chipset to the AGP device.
Fast Write allows the AGP device act like a PCI device. This allows it to bypass the main memory and directly access the data which improves AGP read performance. However AGP write performance is not affected.
It is recommended that you enable AGP Fast Write for better AGP read performance but disable it if any of your PCI cards start acting funny."
"Fast Writes: This setting, whether in the BIOS or in the Forceware Control Panel, is meant to speed up AGP read performance. In practice however it is known in almost all cases to have no noticeable performance impact, and indeed often causes many problems such as crashes and general system instability, especially on overclocked graphics cards.
"My personal testing has shown it to have literally 0% impact on performance on several different setups. Therefore if you are experiencing problems, I strongly recommend disabling Fast Writes in the BIOS (if available) and also disabling it under the AGP Settings section of the Forceware Control Panel for optimal stability with little or no performance loss. " Note that for the AGP Settings screen to become available, you will need to follow the instructions under the Advanced Tweaking section."