First of all, your multiplier is locked on the P4 unless you have an ES (engineering Sample ) chip.
second of all, run your FSB as high as possible as long as it is stabe, but make sure your memory is running at 1:1 ratio because that gives you the best performance. Laster on you can change your ratio and then maybe increase your FSB. Though I highly doubt that your chip will go up to 225 FSB because on the intel 2.4 with 400Mhz bus, its multiplier is locked at 24, and therefor, 225 * 24 = 5.4GHz.
What i think your getting confused about is that, even tho the FSB is User Defined, the quad pumped will still apply so with a FSB of 225 * 4 = 900FSB. So just use user defined, and raise your FSB from 100 to whatever is stable for you.
Ok, we need to know what motherboard you have. If there is a way to fix or lock the PCI/AGP to default speed (33/66), then that's the first thing to do. How high are you trying to set the FSB? With a 24X multiplier, a really high FSB just plain won't work. Try a small increase at first to just see what happens, like maybe 108 FSB (that will give you almost 2.6 gig).