Enabling Fast Writes was originally meant to show an improvement in data transfers that go directly from the CPU to the graphics chip, which doesn't touch such things as "AGP-texturing". 3D-software with very complex 3D-objects requires that the CPU transfers a huge amount of triangle-data to the graphics chip...here is where Fast Writes avoids the stalling detour from the CPU to memory and then from memory to the 3D-chip. The Fast Writes idea is to directly connect CPU and 3D-chip.
The only areas where fast writes seem to benefit an AGPx4 system is on complex geometry scenes with low texture demands, and also helps with increased lighting calculations.
My card and chipset both support Fast Writes (the card also supports SBA), but SBA was disabled in the Leadtek BIOS...to enable it by default (and find out if SBA had any effect on performance), I needed to flash the card with a Nvidia reference BIOS. Running 3DMark, my scores are just slightly lower with both enabled...but in games like UT and CS I gain about 3-5 FPS (not much of an improvement as far as FPS), but in actual game play...I can tell a noticeable difference.