Can I use a PCI-Express Graphics Card on a motherboard currently running an AGP card?

a PCI-e graphics card simply will not fit into an AGP port. it's like trying to screw a lightbulb into a faucet. although there is a new ATI AGP card that's target at guys like you, I suggest you take a look at that one. you don't really have any other options short of buying a new mobo.