Depends what you want to do with your shiny new laptop. Here's my thoughts.
Screen:
Small enough to take on the bike = 10" screen. But you'll probably need to attach a monitor for anything more than watching the results from your bike cam.
Portable and useful = at least 15" screen. OK for short periods but can be tiresome if you're spending lots of time on it.
Desktop replacement = biggest you can afford. If you're not using an external monitor bigger really is better. Bung it on a stand and use a separate keyboard rather than hunching over the laptop for long periods.
Memory:
Minimum=500Mb, typical=1Gb or greedy=2Gb. If you don't have enough memory to run all the applications you've got open the laptop runs like treacle because it's busy using disk space as a memory replacement.
Disk Space:
Don't consider the 8Gb flash option unless you're taking it on the bike. Absolute minimum these days is 80Gb but 120Gb plus is typical.
Operating System:
I'm fairly geeky so I'd install XP and Linux. Mostly 'cos if I'm abroad and logging in through dodgy wireless networks I'd rather be running Linux. For normal folks XP is a better bet.
Other stuff:
Install decent anti-virus software and a firewall before some tosser hijacks your machine and uses it to send out spam (or worse).
I'm hoping to find my way to Soho tomorrow, so you're welcome to pick my brains (such as they are).
Keith.