If you're riding mostly flat I'd say probably 48 x 16 somewhere in that range. Chris King makes a really good headset. Good luck!
I will disagree. That's a pretty tall gear and while it will give you a fast cruising speed, it will be fairly slow to accelerate and tough to slow down (with leg braking). I would go 48x18 for a new rider. This will give you a cruising speed of 21mph (at 100 rpm), decent acceleration, and skid/skip ability.
Once you have monster guads and a good spin you can move up to bigger gears, but I wouldn't start much higher than this.