In addition to psycho's comments check out this article.
AnandTech: The Core 2 Quad Q8400: Intel's $183 Phenom II 940 Competitor
I can run my Q8200 at 3.5 ghz 24/7 if I wished but for 9gb/s bandwidth I need to drop it to 490 cpu /fsb(3.43Ghz).
If the Q8400 needs a lower fsb/cpu ref to reach the same speed it could actually be a downgrade.
ie if its just a Q8200 with a 8 multiplier but wont run any faster than the Q8200 (3.55-3.6Ghz max on air) you would have to drop the multiplier down to 7 again to get max pc performance.
Then again I havent actually tested a Q8400 on "our" mbrd but unless its some kind of $20 swop it would seem a bit pointless.