I personally recommend getting a 500cc bike as your first bike, so a gs500 or a ex500 (ninja500). My reasons for this is that these smaller bikes have a much gentler power band across the entire range of rpm's and are not very jerky. You can learn to enter and exit corners more skillfully I believe on these bikes then getting on a 600 and throwing it around. Also, I do not want to advocate being unsafe and dangerous but I have own a ex500 and riding with more powerful bikes and keeping up with them has definitely improved my skill as a rider. Since I am usually near the back of the pack I can also watch and learn from more experienced riders. At the speeds you should be traveling you really should have no problem keeping up with the bigger bikes, I have taken the 500 to the top of its speedometer on 464. The biggest thing though is to buy what is going to make you happy in the long run, if you feel that you will get bored of a 500 too quickly and it is not worth the investment then go ahead and get a 600 or a 750, just respect the bike. If you respect the bike, and the rules of the road you should be just fine. Remember biking is fun, keep it that way.