I feel like an old man everytime I tell 'newer' riders this, but a 600 is plenty for a novice. Spend a summer learning to ride a 500 or 600 the correct way, then move up from there. I spent a summer on an old 600 Katana, then a couple years on a newer 600 Ninja (a '99 in 2001), and it was an easy transition into a 900cc+ bike. 600's can easily get into the 140-150mph range, and that really is plenty to have fun with, and even push your riding limits.