Did you try to remove the PCI 1394 card and go back to your old setting? If that doesn't fix your situation, then you might have changed something elses when you added your new card. Stupid exemple might be that you disconnect one of your case fan and your cpu now overheat.
Anyway, let me know.
P.S.: 400mhz fsb is very high for long term overclocking. You should expect to have to reduce it as time goes. The change you mentionned here doesn't have anything to do with this tough. All I'm saying is that you will realize after 6 to 12 months that your OC isn't as stable as it used to be. I personally OCed only 600mhz to make sure that wouldn't happen. This is because high OC need high Volt raise which "erode" the electric contact within your cpu prematuraly. By keeping your Vraise less then 0.05V from the original, your reduce a lot this phenomenum. Do what you want with it.
I dunno what the problem was but all better now. Back to my original OC with a 1394 PCI card!
As you suggested, I removed the 1394...returned to my OC settings and booted fine. I simply re-installed the card without adjusting any settings and now I am up and running. Maybe it was seated poorly? It was working though...hmmm.
Your OC warning has me confused though. You said you are OC to 600mhz? I assume that is a typo cus your sig says you have E6600 OC to 3.0+GHZ. I think that would be 333Mhz not 600? 333x9=3GHz. At 400MHz I actually dropped my CPUv from 1.325 (stock) to 1.25. I may drop down to 333MHZ but I don't mind if I need to upgrade my chip after frying it either....