Yes, memory bus should mean FSB (it basically just means the line of communication between the CPU and main memory), but use of computer terminology is never consistent. I guess in SiSoft Sandra FSB means the operating frequency of the CPU, and the memory bus just means the operating frequency of the RAM.
If you have checked the settings in BIOS, under the Advanced tab, and both the CPU frequency and the DRAM frequency are shown as 200MHz, then there's not much you can do.
The only thing to try is to use manual settings. Your motherboard online manual suggests that you can use jumper settings to determine the FSB. On the motherboard, to the right of the PCI slots, just underneath the AGP slot, there are 3 sets of jumpers. Consult the manual and double check the jumper settings for FSB400 (i.e. 2x200MHz). Then go into BIOS and select Manual for CPU, set the frequency to 200MHz, multiplier to 11. Then set DRAM frequency manually to 200MHz. Save settings, and see if it makes any difference.
If you have done all this already, then that's it. Or try another utility to test the memory speed.