7900 GT (I had one) was the last/fastest AGP card released before the switch to PCI-e. I believe it was also released on AGP and PCI-e so you have to make sure of that.
Actually, your motherboard supports pci-e video cards.
I don't know what you're looking to do but a pentium 4 isn't going to play any games from the last 10 years. You could get away with something like Call Of Duty 4 but anything newer than that is going to be a bad experience. Even Minecraft won't play on that CPU.