Based on that article I linked, which Stormy also linked, the voltage their referring to has to do with agp bus signal speed... ie tranfering data on the bus, and not
actual voltage to the card.
Besides, (and this is perhaps the biggest reason you can't fry a card) the cards employ different slots making it impossible to actually fit a 1x agp card into a 4/8x agp slot. There's little chance of frying a card that's sitting on your desk.
The AGP 1.0 specification requires that all implementations support the 1x speed multiplier at 3.3 volts. The 2x multiplier is optional. There's no such thing as a 3.3 volt video card or motherboard which only supports 2x. By default, when the AGP 1.0 machine powers up it selects the fastest speed multiplier supported by both the video card and the motherboard. If they both support 2x then they will run at 2x. Otherwise they run at 1x which is always implemented by all AGP 1.0 video cards and motherboards. There is often an option in the BIOS which allows you to limit the speed to 1x if 2x is not reliable. The AGP 2.0 specification has a similar requirement. 2x and 1x support at 1.5 volts are required and 4x support is optional. The AGP 3.0 specification requires support for 8x. The 3.0 specification isn't as clear as the 1.0 and 2.0 specifications on the subject of requiring the lower multiplier but all AGP 3.0 implementations that I've seen support both 8x and 4x. As a result, you can completely ignore speed multipliers when you're checking for compatibility between an AGP video card and an AGP motherboard. If the video card and motherboard both support the same signaling voltage then there is always at least one common speed multiplier supported by both at that voltage.
Not to mention that you can't find an AGP 1.0 card these days... and even if you could, it would not fit into the board.
I'm really not trying to be argumentative... it's just that I've never heard of frying a card this way.
The OP's board is agp 2.0/4x. The card he's running is agp 8x. The two cards he selected are also agp 8x, so I see no reason why we should fill him with fear about frying the card. Am I missing something here?