Yup, Brainfood, just drag and drop; program windows, icons, even your taskbar.
When you install a second card into a Win98 machine, the desktop 'Settings' tab changes slightly to include a graphic showing both monitors and a "Strech my desktop over both monitors" checkbox. It works like a charm, MUCH better that I thought it might when I first heard about this (underadvertised) feature.
I usually run my little 14" monitor with my desktop, e-mail and a Genome@Home client open, with my main 21" monitor for playing Unreal, or QuakeIII or something. The 14" is running off an 32Mb NVIDIA TNT2 64, but used to have a 4Mb Diamond Stealth 3D quite happily. The 21" runs off a GeForce 3 Ti200.
One thing to note is that (contrary to my previous post), only with the advent of Windows 2000 does this feature allow use of AGP cards :-(