The first video card released on PCI-e was a 6200 or 6600, not one of them 5XXX cards. You my friend have an old out dated system with PCI, possibly AGP, right now i think the best card you can get is the eVGA 6800GT for AGP 8x, if your motherboard even supports AGP.
PCI - Old normal white slots, sometimes other colors, usualy theres 2-6 of them on a motherboard.
AGP - Graphics card slot, only one on a motherboard and they sit a little farther to the center of the board then the PCI slot. It is also different color from the PCI slots.
PCI-E - Newest interface for graphics cards and other various cards. PCI-e comes in different formats, but the slots are usualy very short or VERY long compared to normal PCI and AGP slots.