Do both video cards have to be the same? Do both monitors have to run at the same resolution and color depth?
No and no.
Both cards need to be on the HCL for multi-monitor use, but you can have different res and colour depth on each (not just two) display.
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SuperMicro P6DBS (dual UW-SCSI) BIOS 2.2, 2*Celery 300a @ 450Mhz, 384MB PC100 RAM
SCSI-A=4.3Gb+9Gb, SCSI-B=Tosh32x CD-ROM, Yamaha4416 CD-RW, Iomega ZIP100, IDE1=4.3Gb
IBM EtherJet 10/100 NIC PCI + Nortel ADSL "modem"
Matrox G400 DH 32Mb AGP + Quantum3D Voodoo2 SLI PCI (CL TNT1 AGP on a shelf)
SoundBlaster Live PCI (not Value)
Win2K build 2195 Retail (not 120-day eval)