There are other alternatives. You could consider a video-enabled laptop docking port, such as Lenovo's Enhanced USB Port Replicator (prices ranging from US$109 to $163), or Toshiba's Dynadock (which runs around US$135). These let you add an extra monitor along with half a dozen USB ports to any brand of laptop.
Or if you go by the theory that today's 19-in. LCD is yesterday's obsolete 15-incher, then you could opt for a single larger LCD today and plan to add a similar-sized one in the future. Of course, you'll pay more: 28-in. LCDs start at US$500, while Apple's 30-in. Cinema Display, the Lexus of LCDs, costs about US$1,800. With something that big, you can use utilities such as Microsoft's Virtual Desktop Manager, which let you simulate multiple desktops on a single screen.
Still, I prefer my thriftily assembled trio of monitors. Am I always three times more productive than when I am working in a caf? on my lone laptop screen? No, sometimes it just means I am procrastinating three times as much. But when deadlines rear their ugly head, there's no place I'd rather be than in front of my triple-headed home office.