Dual monitors are nice if you can afford them. I've liked Viewsonic LCDs for video work, although technically you should be using CRTs (or the new hyper-expensive LCDs designed for broadcast) to display the full color gamut for accuracy. It's really handy to be able to put a large timeline across two screens, or keep one for preview and tool selection and whatnot. If all you're looking for is novelty, the one of the large curved screens above would be your best bet. If you're looking to enhance productivity or make it easier to use Photoshop or video software, then dual monitors will most likely be cheaper.
I've used some of the larger monitors, such as the 30" Studio Display, and when I'm using them I don't really feel like I need a second monitor since that single one is so big and provides enough real estate by itself. However, there are times I'd like to have fullscreen preview and still keep an eye on the timeline and so forth, and for that I'd need a second display.
It is my personal preference to have two monitors, but whether it will benefit you or not mostly depends on your workflow and whether or not you want to be looking back and forth between them all day long. I can tell you that everyone I know who's tried dual monitors no longer likes working without them, so if you make the switch, be prepared to stay that way. That will mean upgrading two monitors at the same time (you need a matched set for accurate color, among other things) and giving up more desk space. Is it worth it? To me, and many of my fellow geeks (and business types, too), it is.
My advice is to find someone with the configuration you're thinking of (or even go to an Apple Store or other electronics retailer and ask if you can set up a demo rig) and play with it for a few minutes. See if you like it, and then decide if it would be useful to you.