You purchase the video card based on the available slot on your MOBO. it is typically AGP 8x or PCI-e. SO you don't have much of a choice.
If you are purchasing a new MOBO... go with PCI-e.
the slot should go up to 16x and 32x.. which is at least twice as fast as any AGP slot. So people tend to buy an PCIe card.
Plus people say that the AGP port will be phased out of MOBOs.
I am sure, if a few years, they will come up with some super fast AGP port and will have people migrating back to AGP.
For us editors, the PCIe 16x or 32x compatible video cards can help NLEs, if they use the graphic card's GPU & memory. If your software doesn't take advantage of the card then it probably doesn't matter.
my 1.5 cents...
(someone correct me if I am wrong)
correct, software like the [url=http://www.videoguys.com/avidliquid.html]AVID Liquid 7 will take advantage of the GPU and actually render quicker. Most new motherboards have PCI-E and no longer have AGP. Just make sure before you purchase the video card.