AGP 1x has 266MB/s bandwidth (32bit @66MHz)
PCI-e 1x has 250MB/s (2.5Gbps, 10bit average to trasmit a byte)
So AGP 8x runs @2.1GB/s, PCI-e 4x @1GB/s
At the current time there is no graphics card that can saturate an AGP 8x: the fastests use at most 70%, that is 1.5GB/s.
But PCI-e 4x runs in full-duplex: it can transmit and receive data @1GB/s in both directions, while AGP is half-duplex, so they are virtually equivalent.
Anyway, an 8x slot is required for a master SLI card.