The 400 part isn't exactly your processors FSB... the processor is on a 100 FSB... then it's multiplied 4 times to make 400 for the memory (RDRAM). P4's run pretty hot... things to improve airflow:
Move your IDE cables around... perhaps even round them or fold them in half and tape them so that air can move past them easily.
Add some more fans if you can... for every fan bringing air into your case... have another fan moving the air out (eg. one 80mm blowing in 50 CFM... one 80mm blowing out 50 CFM)
Use some fine sandpaper and smooth the bottom of your heatsink on your processor... and then put some thermal compound on the processor die before reattaching the heatsink... I'd recommend ArcticSilver 2 or 3. But you can also pick up cheap stuff at electronics stores.
FANS FANS FANS... or make the bold leap to watercooling... but for now just focus on airflow