from one designer to another...
my short answer is, if you've got the $$, go big.
you're looking at some big programs running on your laptop: Illustrator, Photoshop, etc., and there's no doubt that you'll have at least 3 of them open at any one time. -- get a good processor. doesn't have to be top of the line, but get something that won't feel slow in a few months.
desktop real estate is all important. you end up w/ so many palettes open in design programs that you'll eat up a good chuck of the screen on a small monitor. and unless you intend on getting a really NICE desktop LCD (i.e. Cinema, very nice if you can get someone to buy you one like i did!), just get a good monitor on the laptop & forget the external. -- go 15" if you can (my 14" original PB is alright, but another 1" would be nice).
you're designing a website: mock it up in Photoshop, cut it up in Imageready (or Fireworks), drop it all into Dreamweaver, preview it in a browser....you've got a lot of stuff going on. not to MENTION video editing! --get as much RAM as you can. none of my computers have less than 512MB.
i've never bought a refurb, but that may not be a bad idea. kind of like a car, by the time you drive it off the lot, it's lost 25% of it's value. and it's not like the new computers don't run the risk of coming w/ issues, too.
hope that helps!
OH, and if you do any kind of video editing, the SuperDrive is a no-brainer. edit, burn, go. sweet.