I am using two monitors w/an Appian Jeronimo J2N card (I think they've recently replaced that model w/a cheap one targeted @ Win98) and it works fine (www.appiangraphics.com). Last December I started running 3 monitors using a Colorgraphic (www.colorgfx.com) Evolution 4 (will do 4); I'm happy w/it as well. Both of the computers run NT. Neither of these cards are absolute bleeding edge 3D performers, but they're not slow either. There's another card called the Twinfinty that is made in England. As of December I couldn't find a AGP multi-card. I know of other people that run 2 Matrox Millenium PCI cards & like that setup, & Matrox makes a single slot multi-card (www.matrox.com). There are several utilities around that attempt to cope w/the problem of dialog boxes coming up split screen on a 2 monitor setup, but none I've seen are perfect. I believe Win98 has native support, but I don't use it;...can't comment. Most of these cards were (Dec) in the $600-$1100 range.
I don't know what use you have in mind, but I use it for interpreting seismic surveys and mapping; if you have several different things that you commonly are comparing it makes a world of difference in productivity. If you use software that has drop down free-floating menus (such as CorelDRAW) it is very helpful to be able to put all of them on one screen not obscuring the image or whatever that you're editing on the other. I will comment that the 3 monitor setup is nice, but I do spend some time just tracking down the cursor.
Hope this helps!