Q:Is PCI Express Video Faster than AGP Video?
A:Yes and No. A 16x PCI Express connection is at least 190% Faster than AGP 8x but this is the connection between the system and the video card. You use the connection the most when your video card is low on memory or when the game you are using uses a Direct X or Open GL feature that isn't supported in hardware.
So, what this means is that in terms of real world performance there may not be a huge difference between AGP and PCI Express if you are talking about identical chipsets. Unfortunately this is very hard to prove because graphics chipsets are designed either for PCI Express or AGP. If you have a card that is available in both forms then you have a graphics chipset that was designed for PCI Express and has a special bridge chip installed to let it comunicate with the AGP bus. The short of this is: if two cards of the same chipset are available in AGP and PCI-E then the PCI-E one will always be faster. On PCI-E you don't have the overhead of the bridge chip so it's faster, and you have the better bandwidth so in intense situations such as high resolution gaming you'll come out on top every time.
The main point here is: If you have a system with AGP on it, it doesn't make sense to upgrade just to get PCI-E video right now. The fastest AGP card to ever come out is likely to be the nVidia 6800GT. If you are at a point where that is too slow then by all means it makes sense to make a complete switch. If your happy with your AGP graphics options, wait until you are ready to upgrade the processor or other components before making the PCI-E switch. For more information on AGP and PCI please see the general FSB guide.