Well the PCI Express is the type of interface for graphics that your motherboard uses.
It is currently the top of the line interface for new video cards, being a step up from the slowly becoming obsolete AGP interface cards.
This basically means that the card fits into a PCI Express slot on the motherboard. The x16 is basically the speed of transfer between the the card and the motherboard. AGP used x4 and x8. I don't know the exact specifics of the x16 rating, but this is a general overall answer.
This post has been edited by Neela: May 10 2006, 05:56 AM