I wouldn't be so fast to bail out on 600+ bikes. You'll find that 650 is actually a comfortable size, although it may not necessarily seem like it at first. 750 isn't unreasonable, depending on how it's balanced. Go to a cruise or a bike showroom and sit on a few (with a friend around) and you'll probably see what I mean. Lean it slowly back and forth (remember when I said to have a friend around...? Now's a good time). If the salesperson comes around to bother you, just send them away. You don't need them for this. If you ever have any plans of going freeway speed...Now's the time to make the decision. My smallest bike is a 550 and it's
okay on the freeway. Go smaller and you may not realize how much you regret it until either you ride a bigger bike or your butt chafes off.
I would recommend getting a classic standard (or a cruiser if you like but I'd recommend a standard for your first because of the dynamics of the rake angle and center of gravity) in the 600-650 range for your first ride (no, I'm not kidding). Four's are nice and smooth, whereas twins which sound beefy and have lots of torque might make you a little uncomfortable at first. Don't be afraid of one that's from the 70's or 80's. As long as the miles aren't sky high, you'll probably get a good deal that'll be with you for years. Just be sure to grab a shop manual and a set of tools while you're out shopping for your riding gear (and I won't harp on it but don't skimp on the protective gear, seriously...)
Don't be afraid to get a bike that isn't in the greatest shape. Chances are you're going to dump it at some point. I've done it and almost every rider I've talked to who has riden seriously for any length of time has stories to tell...whether it's because you were careless with the kickstand or because you paniced in a situation. Keeping your cool isn't a personality trait so much as a learned skill.
Definitely sign up for an MSF course, but do it now as most places they're booked weeks or months. They'll put you on an old beater bike...most likely a Rebel in the 150 range...and move you along pretty quickly but it's a solid course, no doubt about it.
Once you've done that, you're going to want to spend time with whatever bike you bought...serious time. Find quiet roads and ride. Find riders and ride with them, and listen to everything they say even if it sounds condescending. Any serious rider is going to more than happy to talk your ear off, give you a load of advise and want to show you their special, super secret rides and hidden roads. They'll hear and see what you are and aren't doing wrong.
Don't get discouraged or let it scare you. When I started riding, it was months before I felt like I really confidently bonded with my bike and could deal with *just about* anything that came my way. It was probably a year or three before I felt like it was really a part of me and it was strange to not be on it.
You'll want to work on it yourself. It's good experience and you'll be more in touch with what the bike is telling you. Really, a complete novice (with the help of the right tools and a good manual) can pull, completely disassemble and rebuild an engine in a weekend (just be sure you buy lots of snack bags and sharpies. You'll be in a world of emotional hurt later when you realize the obvious in that putting all small parts in a single coffee tin was a bad idea.
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edit: Don't forget to also find the forums for whatever bike you choose, they're a wealth of specific information. But then, the fact that you're here may mean that you'd do that anyway.
