There are three types of slots you can use for graphics cards
PCI - An older standard, only normally used if your system did not have an AGP slot also.
AGP - The "just past current" standard used in the majority of system for several years.
PCIe - the newest standard, in systems from around last year.
So it depends on what type of card slot your system has - your documentation should tell you. If it's over a year old it will alsmost certainly not have PCIe.