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.
While this feature may appear to greatly improve the performance of AGP graphics cards, its actual performance effect is quite negligible. Also, it reduces the overclockability of the graphics card when enabled.
Therefore, if you do not intend to overclock your graphics card, it is recommended that you enable AGP Fast Write for potentially better AGP read performance. However, you should definitely disable it if you intend to overclock your graphics card, or if any of your PCI cards start acting funny.