Also if you are planning on tootling around with your laptop in your bag you might want to consider waterproof/resistance too... Incase you are in a rainy area like me.
I currently use a Camelback pack (yes I drink and ride!
water...) that is very spacious as I can fit my frog togs, personal items, and waterbag with no problems. A lot of Camelbacks also have straps that you can use for a helmet (I haven't tried this yet though).
Just a thought.