When it comes to AGP slots, there are two things you need to be concerned about:
1) The AGP connector (1X, 2X, 4X, Universal, Pro, etc)
2) Voltage that the video card will operate
All 8X AGP cards that I've seen are backwards compatible with 4X slots. If the motherboard has an 8X slot, then you need to check the specifications on the voltage to make sure it can accomodate a 4X card (which the manual/website should clearly state).
2X ~ 3.3V or 1.5V
4X ~ 1.5V
8X ~ 0.8V
Bottom Line: You should have no problem mixing 8X and 4X going either way. However, when you talk about older cards that used 2X or less, then you need to verify the voltage of the card or slot first. Connecting the wrong voltage will easily damage the card and could also damage the motherboard.
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"The secret to creativity is knowing how to hide your sources"
[tab]- A. Einstein