Pretty sure that part of the AGP 8x standard is backwards compatibility with AGP 4x slots. But, I don't know for sure that an AGP 8x card will work in a 1x/2x slot, though.
As Nero said, it may run slower, though that depends alot on how fast the card is. If its a 7-series nVidia or 1X00 series ATI then it may be able to saturate the bus, otherwise the performance difference shouldn't be too bad.
What card are we talking about?