I've been using dual monitors for the past 8 years. Right now I'm using an nVidia card, which can be put into nview "span" mode. (Even a cheap old Geforce MX is capable of doing this) The benefit of this is that the OS sees the two separate displays as one giant display, and applications that are not multi-monitor aware can still take advantage of spanning across both screens. The downside, as jmshepherd pointed out, is that it's hard to read text across the gap. Alot of applications get weird or annoying when they run in fullscreen mode; it's a trade-off you may need to live with. (Although you can usually switch back to single monitor mode on the fly if you need to.) Nview adds an extra "maximize" button to your windows that will maximize an application to one monitor. It's a sometimes ugly OS hack, but it works for Live flawlessly. (Photoshop is another story.)
If I'm working on a small live set, I'll usually maximize it to one monitor and put a file browser and my VST plugin windows on the other monitor. Works pretty snazzy.
Hope that helps you decide!
my mac does this for me automatically. gotta love the mac.