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Signaling Voltages
There are three different voltage levels used for signaling on an AGP bus; 3.3Volts, 1.5Volts and more recently, with the AGP3.0 standard, 0.8Volts. It is important that you know which are supported by your motherboard and your graphics card. Get this wrong and your graphics card won't fit your motherboard or in the worst case scenario may actually damage your motherboard*.
To prevent an incompatible card from being inserted into a motherboard manufacturers use a system of 'keys' and 'notches'. A key is simply some material placed in the socket at a specific position. Unless the graphics card has a notch cut out of its edge connector of the correct size and position the key will prevent the card from being inserted properly.
It is also worth pointing out a general rule of thumb here; any Pentium 4 PC using an Intel motherboard based on the Intel i845 and i850 chipsets only supports 1.5Volt signaling. Therefore you can rule out any graphic cards which only support 3.3Volt signaling on these motherboards.
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