Ok so here is the question:

Any modern computer with an available PCI slot ( not PCI Express, but older PCI ). If any PCI card is stuffed into that PCI slot populating it, it slows down the system vs the system being run without ANYTHING in the PCI slots?

Now I know that this is sort of a trick question because any extra hardware added to slots is going to add to the resource JUGGLE of the system. But the question is mainly if there is a performance hit for having a PCI card stuffed into a system ( lets say for example a Intel Pro 100 NIC ) instead of installing a PCI Express Network adapter into this system.

While long ago you had to worry about use of older PCI cards with newer PCI cards in which the older card would cause the systems PCI BUS to sync at the SLOWER speed killing the maximum performance of the newer PCI card which has to lag to the slower BUS to accomodate the older cards speed, I have never bench marked a system bare without a PCI Card INSTALLED and then benchmarked it with a PCI card installed to see if there is a performance hit at all.

Personally as long as that DEVICE is sitting idle and there is no traffic to it other than sitting idle, I dont think there would be a noticeable difference between a PCI card stuffed into a motherboard and one without a PCI card stuffed into it.

Figured maybe someone else out there has an answer to this question that was handed my way by a person who had concerns that adding an older PCI card to the newer system would cause it to slow down to accomodate that card?

Personally as long as that card is not a Video Card or Drive Controller, I can't imagine any other card that would slow down performance in any noticable way.I'm fairly sure that it is false. On systems that have both the PCI and PCI-E buses are separate- installing a PCI card will not effect any of the other buses in the system.
I don't  know of any modern PCI device that requires the CPU to slow down when it is not in use.  Nor have I even herd of this question before. **

Now if we were talking about mismatched memory sticks, that would be a different matter. Memory modules can a do have different speeds and  such details are documented.

** But somebody did ask something like that on Tom's Hardware.

Does an old PCI card slow down the system/bus? Thanks guys ... printing this out to hand to him.  So this can now be closed.
For the curious, there are lots of links about how PCI works.
This one is good, but has some rough edges:
http://en.wikipedia.org/wiki/Conventional_PCI