All of the P5Q should do 400 MHz FSB without any problems or adjustments at all. Most will do somewhere between 400 and 450 before needing to increase northbridge or FSB termination voltage.
e6400 is 8x multiplier, so that's 3.2GHz.
So this should not be a factor for you at all. The pro has slightly better cooling on the northbridge, but that kind of thing isn't going to be a big factor until you need to push northbridge voltage significantly, which is like 450+ FSB. Most people aren't pushing thosee kinds of FSBs, so it really doesn't matter.
Even the P5QL-E P43 based board I have does 400 FSB fine.