grayracer513 - I don't care what the HP numbers say as I mentiond above my 2006 YZ450F walks all over the new bikes HANDS DOWN!! Again I can go and put an exhaust and keep that performace barrier between the new bikes, so agian why would you buy an 2009 and put 2006 parts in it when you can buy a 2006 taht is already at that great power level? yeah doesn't make much sence to me either.....waht I'm saying is the best stock for stock 450 bike out there is the 2006 YZ450F no questions asked!!!!!!!!!!!
Having owned both an '06 and an '08 I can tell you that the '06 will not 'walk all over' the new YZ's as you say. The '06 has a stronger low end but once you reach the mid and upper RPM the '08 will pull the '06. Not by much, but it will.
Aside from the motor, the '08 has more refined cornering characteristics and the front end holds a line better than the '06. Also, the suspension is just slightly better on the newr bikes. Nothing earth shattering, just refined a little bit.
Don't get me wrong though, the '06 is a great bike and I loved mine...but it is not better by any means.