It sounds strange that SBA would be disabled on the card itself... if that was the case, why would there be an option in the driver to even turn it on? *shrug*
Anyway, you can try cat /proc/driver/nvidia/agp/host-bridge to see what your motherboard is saying is supported, and cat /proc/driver/nvidia/agp/card to see what the nVidia card is saying is supported.
If host-bridge is saying SBA isn't supported, try checking your BIOS setup (get into it by pressing some magic key combination right when you start up, sometimes Del, sometimes F2, sometimes F1, sometimes Ctrl-Alt-S, it just depends on who made your BIOS, and who patched it if anyone).
If it's the card, you may be able to upgrade the card's BIOS to support it, check the manufacturer's website.