I am running an e6420 on an Asus P5K Deluxe at 8X430FSB (1.4Vcore@45C) with 4X1 Gig RAM (4-4-4-12 @2.1V) for a daily driver machine. I did initial OC testing up to 450X8 (1.475Vcore@58C) Orthos stable 12+ hours and 8X455FSB (1.5VCore@63C) stable 4+ hours before I backed things down. I removed 2 of my RAM sticks, lowered the multi to 6, relaxed the RAM timings to 5-5-5-15 and took it just over 520 where my RAM petered out. I have no doubt the board will do more FSB, but I am running PC2-6400 RAM and it doesn't like to OC that much with the rated 2.1 Volts.
I love my P5K Deluxe, but if I was doing it over again, I am not sure I would still buy this board simply because of the cost. If you are just looking to hit 400FSB, any of the P35 boards should get you there. Either the Asus P5K (vanilla) or the Gigabyte P35 DS-3 would be the likely choices for the money. If you want the bad mamo-jamo, by all means, pick up the Asus P5k Deluxe. It looks good. It runs good. It is good. It just costs more.