Don't play with things like FW, SBA and the agp rate. 4x is unstable in a lot of cases and compared to 2x it isn't that much faster (max about 1% or a little more) in some cases it is even a little slower. So just stick with 2x. It was a different case if AGP was totally off or only working in 1x.
There's one thing you can do if you really want 4x. Try to use another agp driver. The nvidia drivers support agpgart (linux kernel stuff) and "nvagp" the nt 4.0 agp support that is builtin into the drivers. Check /proc/driver/nvidia/agp what you are using. Then try to switch drivers using the NvAGP option in the XF86Config-4 file. Check the docs for more info on it.