Mac Pro computers come with three unused PCI-express expansion slots for adding extra graphics cards, RAID controllers, multimedia, and I/O controllers, among other options, to expand your Mac's capabilities. In most Mac Pros the PCI-express slots are set up such that the first two are capable of x16 speeds, and the last two are only capable of x8 speeds at a maximum. Despite these capabilities, all slots cannot simultaneously run at their maximum speeds.
