Suggestion: Pentium M 1.5ghz (or higher) with a gig of ram and do your best to find one with a harddrive faster than 4200rpm (the most common speed). 7200 rpm harddrives will run WAY hotter and you risk frying something, but the speed exchange is brilliantly distinct. Also, the tech that makes the pentium M save battery life won't work if you've got lots of programs running, or are playing a game.
Edit: I recommend a customized IBM Thinkpad. It's gonna cost a lot of money.
Second edit:
I figure you'd probably want to know what I use. I have an Acer Aspire 1660:
http://global.acer.com/products/note..._notebooks.htm
Don't go with Acer. I had to ship my first laptop back because it broke in a day. The second one worked great for about a month before the keyboard went out. I had to ship it to them and they took almost 3 weeks to fix it and ship it back.