As far as multiple video cards in Win2k goes, I've got an old Pentium Pro 200 MHz workstation at school with a Matrox Millennium and a Matrox Mystique (both PCI, 2 Mb, 220 MHz each -- yes, "faster" than the main processor!
) running together with no problems. Both work at resolutions of 1152 x 864 (and higher) without any problems in Windows. The system runs Win2k SP3, and I'm not using any special programs or drivers, apart from the defaults that come with Windows. All I had to do was plug the cards in, and they worked perfectly well.
Now, when I decided that, just for the heck of it, I'd add a third video card, a S3 Trio64V+ (or something like that), it had the same "cannot start device" error that someone else reported, but did not affect the other two monitors (it's out now, since it was just taking up a slot!).
There were some issues, a few years ago about using an AGP card along with a PCI one (in Win98, I think), but that shouldn't be a problem in WinXP or Win2k.
Also, it's
very rare to have to modify Windows' IRQ settings these days, especially with PCI or AGP cards. Do you have any old hardware installed on the ISA bus, if present (modem, NIC, sound card, etc.)? You might try pulling those out temporarily, to see if there are any incompatibilities there.