<< are all geforce 3 cards agp4x? >>
Yes.
<< and what happens if you put it in a agp2x?
will something bad happen? im guessin so >>
Exactly the same as would happen if you put it into an AGP4X slot. Nothing bad will happen, it's completely backwards compatible. Right now the performance difference between the two is about 0-1% so frankly I wouldnt even give a damn if it was AGP4X.... hell I say run it in 2X just for the potential improved stability.