If you ignore everything else I post at least beleive this.
Buy the whole new rig!
Upgrading peicemeal does not work this year due to changing technology. I have a 3 month long horror story of wasted effort.
I should have just bit the bullet and gotten a dedicated PCI-E MB, new vid card (PCI-E) and new power supply (dual rail, SLI certified, etc.), either right away or waited to save up the money if I was short.
As it is I ended up with a giant compromise that will not last as long as I'd hoped performance wise. (Not enough MB slots, power supply on the edge, inadequate vid card upgrade.)
PS I don't think the 7800 or X1800 families come in AGP slot versions. Top end cards are reserved for PCI-e now.