As for getting the AGP to work without a whole new motherboard, it's not possible, as it's a whole different expansion slot.
There's a good example of the PCI-E versus the PCI. If you have to have PCI, those cards would be okay, you'll probably be able to play newer games, but I'm not making any garuntess, but your not going to see major upgrades. Your best bet is to get a new Motherboard, CPU, and Graphics card, but I don't know if that's really an option or not so yeah. Any other quetions, just ask away.