Yes you can convert it by fitting a motherboard, sorry, couldn't help myself.
I presume you have a motherboard that doesn't have a AGP slot? and all PCI graphics cards are slower and more expensive than there AGP counterparts.
If you are serious about a decent gaming card (minumum
GF4 Ti or Radeon 8500 or above) then it won't work out that much more to source a cheap motherboard "with" an AGP slot and buy the faster BUT cheaper AGP card.
Martin
Replying helps further our knowledge, without comment leaves us wondering.