Reading the wiki compatibility section on AGP, it seems that it can still be a bit of a minefield, that some videocards incorrectly have dual notches.

One of my non 3dfx cards is supposed to be 4x, but it has got two notches, so it does fit in a 3.3 board, but I get a completely red display on a AGP 2x board, but the previous owner tested it as working.

Should I give it a try in a 4x board?

This actually always confuses me lol, most cards up to the 6800 Ultra are supposed to have 3.3v backwards compatibility, but sometimes it's a bit weird, cuz the cards aren't really compatible sometimes.

You can measure it using a multimeter. For details: thandor.net - Is my AGP 2x/4x card 3,3V or 1,5V? :).

That's excellent, never knew that. Indeed it's 3.3v then. I tested it on two boards, but on both the card was giving a red signal.

Thanks for the link. :)