If the guy's having that many freaky problems, I seriously doubt it's all related to having dual monitors.
I started out with an ATI Xpert 98 PCI as my secondary display, but had so many problems in Win2k that I gave up on it. Then I got a G200 from uBid for $63 (quite a bargain at the time), and it has been perfect. I love the convenience. I do web development as a hobby, and it's really cool to have CF Studio open on one monitor while previewing the page I'm building on the other without having to switch windows using the taskbar. Multimonitor is one of those features that sounds extravagant, but really comes in handy.
The only game-related problems I've had are with OpenGL games, Q3A specifically. It won't start unless the secondary display is disabled first. Otherwise, no complaints.