Right, the PCI version is perfectly acceptable, however the AGP version of the GeForce cards require the second revision of the AGP port wich has a different voltage requirement. You can damage your motherboard by plugging a card with too much draw into your AGP slot . . . it may not even fit, since they changed the shape and pinout between AGP 1 and AGP 2.
In otherwords, you will have to go with a TNT2 for AGP in the most likely case, if you even have one. The PCI upgrade is the one most likely to help you.