To be truly "Vista Capable", your computer needs to have a pair of GeForce 90500 cards...and seeing as how the 8800 was released not long ago, I'd say you have a bit of a wait ahead of you =)
Just kidding...I ran Vista (RC1) on a GeForce 6600 for a while (I only got a 2.8 out of 5.0 in Vista's "user experience" rating, so if you're looking to do better, then SLI GeForce 7300's or better would be the ticket). As for PCI, well...I think you're limited more by the bus than by card selection. The don't make GeForce 8800's for PCI...bus can't keep up.
So if you can upgrade to AGP, I'd *strongly* recommend you consider it seriously.