Don't worry about "not wanting to buy another bike right away."
You don't "outgrow" bikes so much as you get used to them, and want something better. Besides, the market demand for a used 250/500 sport bike is HUGE. Resale is a piece of cake.
Start smaller, a CBR 600 is nothing like a cruiser, it's a full-fledged sport bike. You ARE going to make mistakes learning how to ride, and you're much better off making those mistakes on a Ninja 250, Ninja 500, Suzuki GS500(check out the new one on suzuki.com...good looking bike) then you are on a 100+hp bike with sensitive throttle, brakes, and handling.
If this means anything to you, I started on a Ninja 250, am now on my third bike, and I actually wouldn't mind getting another Ninja 250 for a second bike to ride locally. They're very fun to ride...and the light weight will help you build confidence faster and become a better rider than you would starting on a bike you consciously have to "take it easy" on.