If this is your first bike, don't go with the S4R. Even if it's your second. I will say (and pretty much everyone else will too) to stay lower in the cc's. Or get a beater to practice on. Don't go big for your first bike. Good luck with your decision though and welcome!
A 600 can get you in just as much trouble as a 996... take the MSF course, since you already have dirt bike experience, you'll be learning what it feels like to be on pavement. (where you should/shouldn't brake, what it feels like to skid on pavement, etc)
Buy the bike you want, no matter what you get, you'll be a newbie, so respect it and ride within your limits.
As a relatively new rider (little more than 1 yr) - I can say that getting a midsized bike isn't a bad idea. I have heard a lot of stories about dumb mistakes end in your bike laying on the ground, but luckily I have yet to drop mine. I very quickly became acclamated to my M750, and I would say an 800 is also safe. I'd probably pass on the S4R this go around just because its a lot of bike and a lot of cash.
I think inexperience gets you in a lot of places that engine cc is independent of -- underestimating corners, taking them too fast, or overestimating your cornering prowess; also having the proper reaction to surprises or changes in traffic while on the road becomes more natural with experience.
If it is your personality to get on the throttle, then you should be cautious about buying an S4R because you WILL test the limits. hahah. Or if you are more modest and safe and you can control those urges until you've got some riding time under your belt, maybe you'd be fine. I just would cry if i had an S4R and committed a newb mistake like getting off the bike and forgetting the kickstand or something.
I'm on my first "real" motorcycle. I've put 2000 miles on my '05 M620 in the past 5 weeks. Previous to that I was on a '94 Suzuki GN125. A street-legal bike that could almost go 58 MPH. I'm a speed freak. I have NO patience. The 620 is more bike than I can handle, and I'm having a bit of trouble understanding the need (not desire) for more horsepower. I can INSTANTLY pass anybody at any speed. The bike is bone stock and I took it to 125 MPH yesterday and there was a bit more power left over. I do realize that everybody says "you're gonna want more power in a year" to everybody with a bike smaller than a 750, but just ask yourself: Do I need to do wheelies?
When you were ridding the quads and dirt bikes did you everthink to yourself 'Man I wish that this thing would go faster!' or were you content just to be on the bike? I'm new myself...the OD just turned 4000 miles on my 620. I know that I'm the type of guy that will never ride my 620 to it's full potental. But ever time I ride it I always have a huge smile on my face. After a short ride on my brothers 999 I was very intoxicated by its power and speed but still very happy with my bike, because I know my limitations. Let that be a guide.
As far as the color goes get a bike and paint it blue and white then get some high pipes. When your done with that RIDE!
Wow, this is really odd. I originally posted this over one year ago, and it popped up today. Thanks for all the advice, but to bring conclusion to the story I ended up with an '02 750. Love the bike and have absolutely no regrets. Fast enough for me any day of the week, and still can't take my eyes off it every time i go in the garage.