What is the difference between PCI-E and AGP graphics cards????

AGP and PCI-E refer to the video bus type, or method of transferring information to and from the video card. PCI-E is the newer and faster method and x16 just refers to the bandwidth of the bus. Any new motherboard will be PCI-E but if your computer is older (5+ years) then it will probably be AGP and the two types of cards are not interchangeable. If you have an older computer you will have to buy AGP or buy a whole new motherboard which will include buying a new processor and RAM as well.