A PCI-E(xpress) card will fit into a standard PCI slot on any mobo...however it will not work!
Its kinda like pluging an AGP x 8 into an AGP x4 slot, sometimes the card will work fine, but you will not get the benefits of the extra x4...other times (in really old mobo's in the case of AGP x it will not work at all.

PCI-E is alot faster than AGP and more stable when overclocked, however unless your mobo supports PCI-E stick with an AGP card as they are faster than standard PCI.

Edit: As for the PCI slot question about what you can plug in to them, well....

USB Card
Firewire Card
Modems
Network Cards
TV Cards......the list goes on and on