It really just depends on what else you have going on your PCI bus and partly how much data needs to get passed to your card. You will be limited to 127.2 MB per sec on the PCI bus to the graphics card. Now with the card having a decent cache on card, it is going to depend on how much graphics data is needed that is not able to be kept in the cache on the card.
An 8x AGP port will allow up to 2034.4 MB per sec to be transfered to the card, BUT I do not believe we even come close to that number even in the most intense games. In fact, I do not believe that much more then 2x AGP is really utilized by graphics cards that have a decent amount of on-board memory. The whole point of that increased speed was to be able to utilize the system memory for your graphics card memory and not have any memory on your graphics card. This was because integrated video could then get a decent performance boost by having a system memory set asside (set by the BIOS setting called "AGP Aperture Size") .
Again, the reason for that huge boost in bandwidth to the cards were for allowing the memory to be removed from the cards. It was a good idea for the low end systems, but for higher end 3D systems, it just wasn't good enough due to the latency involved in this type of setup. Having 128MB of onboard video ram makes a huge difference in the amount of texture lookups and effects processing. Thus limited the need for the large bandwidth for the card.
The reason we are now moving to PCI-express is to have a system wide bus replace integrated into the motherboard. This bus is for other devices as well, not just a graphics card only like AGP. We didn't need it for the higher bandwidth to the cards, but for the overall higher bandwidth of the system itself. PCI-express 1x slots allow 250MB per sec bandwidth. Well it is really 500 MB per sec if you count the fact that it is full duplex unlike AGP or PCI which is single duplex (in otherwords the PCI-express slots can communicate in both directions at once 250MB per sec to the card and 250MB per sec back to the North bridge/South Bridge/other pci-express slot).