I would say to look into IBM's ThinkPad T40 or T41. While they may not look the sexiest on the planet (compared to Apple or Sony), they are some of the most rugged laptops available on the commercial market. They can also be configured to well above your required specs. It will cost you, though it's WELL worth it according to everyone I know who has one.
I'm about to replace my aging Dell Latitude, and while I'm budget shopping for a used laptop under the $600 mark, everyone I've asked or talked to has recommended the IBM ThinkPad T20 (usually T21 or T22, though I did find a T23 I am willing to spend $40 more on).
ThinkPads have been the highest (if not the ONLY) recommended line of laptops to look at on numerous other boards.
Reguardless of what you decide on, STAY AWAY FROM "HOME" MODELS, such as the HP Compaq Presario (look at the Evo line) or the Dell Inspiron (Latitude would be where you want to go here). The plastics used in the business class laptops are generally better. In Dell's case, for the support (should you ever need it), you talk to someone in Texas versus the home class support, which is routed to India. Other's have consideration this alone to be worth the extra investment.
Hope all this helps some and is coherent!