AGP vs PCI-E
AGP is the older standard in the the video card industry and PCI-E the latest and so far the most effective interface with the advantage of having a high bandwidth than AGP. The reason behind in this is that PCI-E uses what is called 8bit/10bit encoding, which means that 8bits of data are transmitted into a 10bit transmission maintaining signal integrity. PCI-E design is a serial bus configuration that is able to transmit data simultaneously in both ways. AGP 8X can only transfer 266MB per second in PCI mode, and in spite of what transfer mode it takes, AGP is never as fast as a x16 PCI platform. PCI-E platform offer benefits that AGP can not deliver that is why vendors have shift from AGP to PCI-E.
An important feature of AGP is direct memory execute (DIME), which is capable of accessing the main memory directly for texture mapping operations through pipelining and sideband addressing. Pipelining is a technique where multiple instructions overlap in each execution. Sideband addressing is a technique allowing the data to be introduced to the data bus simultaneously with the data being sent. In pipelining, AGP makes multiple requests during bus memory access while PCI only makes single data transfer and waits until that it is been transferred before it could request another transfer.