OK, important question for me- stupid one for you guys

Im in my 30s, weigh 160 @ 5ft10in., NEVER RODE A DIRTBIKE IN MY LIFE BUT ONCE, I ride very aggressively on a quad(I can stay with 85% of the local dirtbike riders on practice days at the local MX tracks and hit the 60fters too), I am athletic and in very good shape especially for my age, I mainly want a bike because Im very limited on were and when I can ride the MX tracks because of the obvious reasons, Im a thumb pusher but love MX racing in general. Ive just always owned quads mainly because my parents would never buy me a bike of any sorts when I was a kid :(
I NEED A BIKE I CAN LEARN ON BUT NOT BE UNSATISFIED WITH A YEAR FROM NOW! I prefer Honda and Yamaha. Should I get a 250 or a 450 thumper or should I get a 125 or a 250 ring dinger? Thanks ;)

BTW, IM very mechanicly inclined and actually enjoy tinkering with my rides. Im used to premix or race gas so thats no biggie either. Im wide open on what to get, which I will buy new, just dont know what to get :whoa:

Thanks fellas, Im gonna get the '07 YZ125 :ride: