No offence, bones, but although fatboys are one of the coolest bikes ever made the power comes on much more smoothly than a rocket with 1/2 the cubes. I cut my teeth working at Buell Motorcycles and Erik convinced me (easily) that alot of rideability power wise has to do with the number of cylinders. Thumpers like to "pulse" with each thump which allows the tire moments in between firing to regrip. An inline 4 on the other hand, will just "squirt" you forward , which is fun as hell, but the back can slide out before newbie reflexes know what happened. This is why Honda has built an inline 4 that fires all 4 at once, then regrips when the other 3 cylinders would fire on a normal bike, and fires again. They call it a "Big Bang" racing engine. Not recommended on your bike! (Unless you like rebuilding motors.

So I recommend going with a twin. I'm biased, but aren't we all? The above-mentioned
SV650 is a "starter" bike with balls! I've seen guys pop up the front wheel at about 35 MPH. That's plenty o' power in my book. I haven't ridden one, but the numbers and a walk-around imply great handling as well.
My first bike was a Shadow 500 (cruiser) which I loved. By the way, after riding with the guys at Buell for a couple years I almost pasted a Ninja I assumed could hold a corner as I had about 10 MPH on him as he drifted in and out out the groove. Most of these rocket riders are great at wheelies but couldn't drop a peg smoothly if their lives depended on it. Only the rarest rider uses the bike to it's fullest potential. Most bikes will go faster than we should rightfully go on public roads anyway. Ask a seasoned rider (8-10 years) and he or she will tell you fucking around on the street is for assholes. Take the MSF courses (they'll lower your insurance too) and if you want to go fast, take a weekend racing class at your local track. Consider this when buying your bike, as there may be minor mods required. Mostly removing bulbs and taping guages.
I laid down the first time after riding 8 years.
Riding at night on unfamiliar roads while intoxicated is BAAAAD! (blushing) Most wrecks involve the overlapping of risk factors, so watch out for that...If you do fuck around on the roads, for god's sake do it in the country, away from road signs, trees, and poles. I met a gentleman who flipped off the curb into a sign and lost the limbs on one side of his body. Don't know why he didn't bleed to death.. Choose your corners wisely and check-ride them before running them faster.
Oh-yeah first bikes-

I also have ridden a GS750, Buell Thunderbolts and S1's, GS1000, and currently ride an old 81 Yamaha XV 920. I
prefer the feel of twins, but really each bike has it's virtues. Start out cheap and after riding a while you may decide that you like a bike you didn't expect to. That's happened to me more than once.