The major difference between AGP and PCI is that PCI, or peripheral component interconnect cards, only have access to the inbuilt memory on the card while AGP, or accelerated graphics port cards, have access to the system memory in order to assist with complex operations like texture mapping. An AGP slot gives a video card direct access to the processor via a dedicated bus, providing it with more bandwidth than the PCI.
AGP slots also have a dedicated point to point channel that provides the graphics controller with direct access to the main system memory when processing graphics. These enhancements allow the AGP video card to perform much faster than a PCI card.
Another difference between the two is that PCI is a bus while AGP is a port since it connects two nodes. PCI and AGP also differ in terms of the minimum alignment and length requirements for transactions. PCI transactions are aligned on 4 byte boundaries and must be multiples of 4 bytes while AGP transactions are aligned on 8 byte boundaries and must be multiples of 8 bytes. PCI and AGP slots are different in size. This means PCI cards cannot fit in AGP slots and vice versa. Each of the graphics systems has its own dedicated connection.