you dont need a 450. im 13 5'6'' 105lbs. and i ride a 125. sure i can get on a 450 and ride around in 2nd gear. but why when i can turn faster,jump further and do anything better on my 125 in 4th gear. its not all about power. its about your ability to use the power.
but kids these days dont think of what they need, they think of what they want.
personally im looking at either buying the 09 250f or 450f right now. but i have a yz125 now so i know how to handle a bike and i've also got a trx450r so i know how to handle the power.
you crash on that 450f and your toast man. quads are 100X more stable than bikes and i've come close to ending my life a few times on my trx, which isnt nearly as powerfull as the new yz450f's.