As you may already know, AGP is basically PCI on steroids. SBA enables the sideband address port. One limitation of PCI is that when a request is made to access system memory, no other requests can be made. Sideband addressing removes this limitation.
To enable it, you may edit the line in the os-registry.c file containing "NVreg_EnableAGPSBA" to 1, or you can enable it when loading your nvidia module. I forget the insmod / modprobe syntax, but I think you can just add the NVreg_EnableAGPSBA=1 as an argument like this:
insmod nvidia.o NVreg_EnableAGPSBA=1
Toy around with it if you get the chance, I'll check later on myself and post instructions if I figure it out.
"Floating in a dream-like state, I am the emporer of a parallel universe." --Arch Enemy
My xorg.conf file