This is a very good question that I wanted to know the answer to, as well. So I asked uncle Google, and found this:

Quote:
Will my 8X AGP video card work in my 4x AGP slot? This question is not answered as easily as it might seem. First let's establish a few things you need to know to answer this question.
What is AGP 2.0
AGP 2.0 is a 1.5V AGP slot capable of 4x,2x and 1x speeds.
What is AGP 3.0
AGP 3.0 is keyed like a 1.5V AGP 2.0 slot, but it only uses 0.8V of power. It supports 8X, 4X, 2X and 1X speeds.
Rule #1: Speeds are backwards compatible
AGP Speeds are required to be backwards compatible. This means that an 8X video card MUST be able to run at 4x, 2x, or 1x speeds. However, this does NOT necessarily mean that your 8X card will run on a 4x slot.
Signalling Voltage incompatibility
Because speeds must be backwards compatible, Signalling Voltage is where incompatibilities arise. Basically:

* All 8x cards are 0.8v AGP 3.0 spec
* 8x cards will fit in 1.5V slots, and can tolerate the voltage, but will NOT run properly
* 4x cards can be either 1.5V or 0.8V
* 2x and 1x cards are either 3.3V or 1.5V
* Except in the case of the 8X cards, using the wrong voltage card with the wrong motherboard can result in damage in card and board
* If you install a card of incompatible voltages with the motherboard's specs, the card will NOT run

The connectors on AGP video cards are keyed in such a way that you can only install equipment that have compatible Voltage keyed connectors. Normally the key of the card determines its signal voltage. AGP 1.0 and AGP 2.0 cards using a 1.5V key will signal at 1.5 volts. However, AGP 3.0 devices can tolerate 1.5V - they won't be destroyed, they just might not work properly.
The Bottomline
The bottomline is that your 8X video card can theoretically be safely TESTED in any 1.5V motherboard for compatibility. But if you know for a fact that your board only runs 1.5V AGP 2.0 spec and your 8X AGP 3.0 card runs only 0.8v spec, then the two should NOT be compatible one another. At the very least it will be unstable, if it runs at all. However, we've had a lot of users tell us that their AGP 8x cards work on 4x only motherboards. This is likely due to the fact that some AGP8X video cards are in fact universal 1.5V capable AGP3.0 cards that can run on either 1.5V or 0.8V (remember, AGP speeds are backwards compatible, only voltage incompatibilies cause problems).