Go with the dual set up
Letterbox
Being a print and web designer I can say from experience that the dual monitors will be a godsend when it comes to design work. I have a 23" Cinema Display/22" LaCie Electron Blue setup at work, the Cinema Display for my main documents, the LaCie for palettes and for color correction in PS (a great CRT still beats a great LCD for color fidelity). I use all the big design apps daily: Illustrator, Photoshop, ImageReady, InDesign, QuarkXPress(ugh), and Dreamweaver. InDesign in particular, at least until CS came out, was a serious waste of real estate in regards to palettes.
I highly recommend an LCD/CRT combo if serious color correction work is being done. If it isn't, you're probably fine with the 2 LCDs. Apple uses really good quality LCDs so my Cinema Display is actually fairly accurate color-wise. But that's also why you pay more for a single 20 from Apple than two 17s from someone else.
Believe me, if you have the physical room for a dual monitor rig, you should do it. I hate having to come home to a single monitor, even if it is a 20". Hope this helps...