3Dfx used SLI over PCI only.
No one used multi-GPU with AGP (outside of a lab anyway)
NVIDIA reintroduced SLI over PCI-Express. AMD followed suit with CrossFire.
There were isolated instances of people using modified drivers and hardware hacks to force SLI between the NVIDIA 6000-series AGP and PCI-Express cards, but I think that's only because cards like the 6600GT were built from the ground up to be PCI-Express and the AGP models were identical other than the AGP bridge chip. Anyway, based upon links I can find today it all came out to be little more than a proof of concept at best, not something actually usable.
If you want the best single AGP graphics available, look for an ATI/AMD HD4670 or HD3850.