If there is no way of adjusting this manually in the bios, then the system is obviously auto-detecting the FSB for the memory.
What is puzzling is that the memory now appears to be running at 333MHz, rather than at the FSB of the slowest ram module.
Its almost like its set the memory FSB at the average for the two modules.
To be honest, now that I realise that your original module was running at 400MHz, I would advise that you buy another module (PC3200) instead of using the PC2100. Apart from the fact that the PC2100 module has slowed the overall memory FSB down, because it is running at 166MHz, the older module is actually being overclocked. This is likely to affect the stability of the system overall.