400x10 is very doable, I am doing it right now. I'm coming from an G0 X3230 that did 400x9 stable, and I don't regret it.
Remember that a 4 ghz yorkfield is like having a 4.4 ghz kentsfield, so you are really gaining nearly 800 mhz of performance clock vs. clock
Bottom line though unless you are doing this for sheer enthusiasm, gaining a few minutes at encoding isn't going to change your life, but you will be 800 bucks short (assuming you sell the Q6600). Is there something else you want to tell us about your encoding endeavors?