Help is here!!
Revision 2.0 boards support 400MHZ FSB. Anything less only supports 333MHZ (200/166 FSB in the BIOS setup respectively). If you don't know, check your box. If it doesn't specifically say revision 2.0 or ULTRA 400 anywhere on the box, it's probably the older 333mhz model.
Most importantly, your DDR RAM should be 400mhz (PC3200 or higher), your CPU should be a Athlon XP chip that is designed for 400mhz, and obviously, your Asus board needs to be 2.0 to reach that elusive #. If anything is less, be prepared to experience instability or fry something.
If you do have an older board you can still hit higher levels, but usually not 200 in the BIOS (the FSB is effectively doubled which is why 200=400 and 166=333). Maybe you can hit something like 175-185 and still get a nice jump in performance.
If you need more specific information, go to nforcershq.com and browse around and/or sign up for their forums, Asus specifically.
You can also check the motherboards forum at anandtech.com as there is a huge Asus thread there. nforcers is probably easier.
Good luck!