Well. . it's not that much different then general computing. Depends if you'll be stuck with an IDE or not (Netbeans, Eclipse, ZEN studio). If so, you'll need 2 or more GB of RAM, otherwise the biggest need is CPU speed followed by screen size / resolution.
I prefer to go with a 17 WUXGA, but 15.4 - 16 would be fine as well. at least WXGA+ (1440 x 900) . Or any stnadard resolution and you use a docking station with the monitor or monitors of your choice.
Last on the list of priorities is graphics.
I consider the T500 to be the current gold standard for this type of application.