Yes, some 775 socket motherboards and some 939 socket motherboards have one or two PCI-E slot(s). As of now, PCI-E is only used for video cards. If you have two PCI-E slots, they would be mainly used for SLI (two video cards).
Most PCI slots are white (there are some brands that make slots the color they want).
Here is a motherboard with PCI-E
This one is more complex. As indicated, it has 2 PCI slots in the middle. The 3 shorter slots are PCI-E X1 used for Networks cards... I'm not sure what other use it has now. The longer slot is the PCI-E X16 that you would use for a PCI-E video card and is closer to the side of the board.
Another difference between the AGP and PCI-Express is the gap in the slots. This gap on the AGP is closer to the right if the board is positioned like in the picture. The PCI-E gap is closer to the left.
If you want you could tell us what model is your motherboard and we could tell you what slots you have...
The photo PCI slots look the same to me, but not the cards certainly do not. I found by looking at packages with windows, I could tell which ones would fit or not, also comparing to the modem card to see if they match, the PCI and PCI bus are the same type, the PCI Express is faster and physically different.
I found I bought an AGP 4x after they told me any computer a year old would likely by AGP... I found I had already bought an AGP off eBay that I had been stuck with as it didn't fit any of my slots, so my eMachine needs a PCI/PCI bus card as well, I'd imagine. I replaced that machine's Celeron 700 with a Pentium3 750MHz, but could use a higher speed to do video captures, etc.