Going to 4strokes.

Hi.
Yesterday i blow up my 87' kawi at nationwide while being in 10'th position,and thought that no more 2stokes for me,guz 4'rs are a lot faster than smokers for they're awesome low-end power..
So i have looked at:
Yamaha YZ250F 04'/05'
Suzuki RM250Z 05'
Yamaha YZ450f 04'/05'
Honda CR450F 05'

So maybe pros and cons for these bikes..

1) your physical size:
* 6'3 and 176lbs
2) How physical / aggressive are you:
* very aggressive.
3) what do you plan to ride:
* MX tracks all the time.
4) Do you have any riding experience:
* 1Summer on mx tracks,6years in woods.
5) Do you think you will race:
* Yes,for certain.
6) Are you mechanically inclined and will you be doing your own bike work:
* All works by myself.
7) Do YOU have a preferance to a brand/ motor choice (2 or 4 stroke)?
* 4Strokes nowadays ...
8) Do you have a dealer close by your home that you might use and what brand(s) does he carry?
* Yes,but gonna buy used bike.
9) How much ($) do you plan to spend on a bike?
* About 3000$..
10) Do you live in California?
* No,Estonia (Eastern-Europe) ;)
11) Your age?
* 16
12) anything else that you think would help form an opinion
* -

What about older yamu's ? 04' and 05' yz250f/yz450f?
What would be better for my size on rider? 250 or 450?