@Jose:
Other cards could be used yes, but they have to support the signalling levels specified in the AGP 1.0 specification. All AGP-cards constructed according to the AGP 2.0 specification (those cards supporting a maximum of AGP 4X transfer mode) should support AGP 1.0 signalling levels, but the new cards constructed according to the AGP 3.0 specification (those new cards supporting AGP 8X transfer mode) will are not required to support AGP 1.0 signalling levels as it is not a requirement in the new AGP 3.0 specification.
But backwards-compability has always been a strong card, so it is not impossible that many cards are backwards-compatible even if the new AGP 3.0 specification doesnt require it.
There is only one way to find out.. or more than one way.... anyhow... the safest way is to plug the "graphics-card to test" into an old K6-II motherboard supporting AGP 1.0 only (boards supporting maximum AGP 2X transfer mode) and see if it works. If it works there, it should work on a Amiga-PCI-busboard with the AGP2PCI adapter. Another way to test it would be to plug it into a Amiga-PCI-busboard using the AGP2PCI adapter and running an utility to search the PCI-bus for it... I think those utilities are available to all PCI-busboards for the Amiga.
Ofcourse as Jose said you also need drivers written for a card not yet supported by drivers on the Amiga, but before drivers are written it is quite essential that the graphic-card works on the hardware-level

.
/Patrik