How often are you going to be on the road ?
I have a 17" Dell Laptop for OOO dev work and to be honest, it's a pain in the ass to drag around and extremely unportable.
That said it's a 1920 * 1200 res screen, 4Gb Ram, 7200 RPM disk. I've tried using Visual Studio on small res (1280*720) screens and I just loathe the experience and lack of screen realestate.
Decide on whether you want to go 13" / 15" / 17" and do a pro's & con's list of portability Vs. dev experience.
After that, max the machine out in terms of RAM/CPU/HD Speed to the whatever your budget can afford.
Edit
Just to re-iterate some other good comments made.
If an SSD is an option for storage, go for it, though these tend to be quite expensive add ons.
4Gb of RAM should be the standard.
And for comparison, dell categorises it's laptops in terms of weight/portability as:
- Less than 2.2 Kg (Light)
- Between 2.2 - 2.8 Kg (Medium)
- More than 2.8 Kg (Heavy)