Don't worry, lots of people get confused between PCI-X (or more commonly known as PCI) and PCI-E.
No, a PCI-E card will not fit into a PCI slot. They're different sizes, and like TRDCorolla said, they're completely different technologies.
Infomation for PCI-Express:
Information for PCI:
Like I always say, "When in doubt, always use Wikipedia."