Well to use pci express gfx you would need a new mobo so you will have to allow for that.
The slowest pci-e card will far oustrip your current card.
However ATI recently announce they will be converting their pci-e chipset to work with AGP as the AGP market will remain for quite some time to come.
PCi-e is in it's infancy with driver problems and others so I would suggest that unless you are an extreme benchmarker that AGP will do you fine.
What is the rest of your system? You may find that any GFX upgrade is bottlenecked by your cpu.