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 without PCI-E
[IMG]http://images.tigerdirect.com/itemdetails/A455-1056/A455-1056COUT.jpg[/IMG]
You can see there are 5 PCI slots. On top of them is an AGP slot (usually brown and located closer to the middle horizontally).
Here is a motherboard with PCI-E
[IMG]http://images.tigerdirect.com/itemDetails/A/A455/A455-1052/A455-1052-callout.jpg[/IMG]
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...
Pictures linked from www.tigerdirect.com