Any card not using PCI-E 1.0/2.0 is not likly to do well in modern games. They might have umpteem billion transistors and frequencies never heard of before, but still will have problems running games well. Do not underestimate the traffic between CPU and GPU.
Every render instruction has to be sent from the processor to the GPU through the interface. On top of that all textures have to as well, and god help you if you run out of GRAM as then all the texel information has to transfer on top of that which will really grind your visuals to a halt. As you guess PCI-E is 2 times as fast as AGP*8, atleast. On top of that most modern cards are PCI-E 2.0, which is another 2 times as fast as PCI-E 1.0.
All in all I am saying AGP 8* will bottleneck your visual preformance, no mater how big or powerful a card you stick in. Thus why nvidia has not made any recent cards which support AGP.
BlargHonk, the card you linked is also PCI-E 2.0, thus if he can use one he certainly can use the other.