can some one clear this up for me, ok i have a p5n-e sli asus mother board and i was wondering if i could run a reguler pci express card on it or does it have to be x16?????????? i tryed a pci-e and dident work. pleas help

Here's a line in Wikipedia
"a smaller card can be used in a larger slot"
PCI Express - Wikipedia, the free encyclopedia

Basically it says you can use a PCI-E x4 or x8 in a x16 slot, but not the other way around.

so i could put a regular pci-e and run it on a motherboard that only takes x16

Yes, if it's a PCI Express card.
The older PCI will not work in a PCI Express slot.
The X16 (by 16) means it has 16 lanes. So a x4 (by 4) or a 4 lane PCI Express card will function in a x16 slot and use only 4 lanes.