If your not looking into racing just yet then the best option would be a 400 class machine like the Z400. They are a better suit for the trails and recreation and there are hundreds of aftermarket compaines so when your ready to race then your limits are just your pocket book.
I second that. LTZ400 or a TRX400 make great trail bikes out of the box. If you must have bigger, the Raptor 700 is a good trail machaine (more power, less handling).