I have a nVidia GeForce 6800 (128 MB ) AGP 8x .
Will this card work on a M/Board with PCI Express 16x ?
Can Anybody please tell me.

Pci and AGP are different slots, the cards don't interchange. Therefore the PCI level of the motherboard doesn't concern the AGP level. Find out the speed of the AGP slot on your motherboard.

Here is a link to a useful page on the types of AGP, basically you have to have the voltages correct!

http://www.ertyu.org/~steven_nikkel/agpcompatibility.html