First of all Fast Writes and SBA can be very unstable on some chipsets. (not on all anymore)
For stability reasons these options are most of the time disabled. To use them you can load the kernel module with special options. Like the one you used (NVreg_EnableAGPFW=1). You can also edit os-registry.c of the kernel module source.
For both ways you need to load the kernel module and then startx. After a start of X agp is initialized.