First, most PCI cards will not operate as secondary cards - the PRIMARY INIT DEVICE must be set to PCI (from AGP) in the BIOS, which means that the boot sequence will show on the secondary monitor, but you can change the primary card to the AGP card in Windows.
The Matrox G200 is a popular card for running as a secondary card for this reason. You can usually find a PCI version on ebay.
Other good cards are the ATI Rage128 and some nVidia TNT cards (not sure which ones or if all are OK)
Craig