Lemme get out my crystal ball and give you the answers you seek ...
Truth is no one knows what is going to happen in 3 years in the computer industry, much less next year, barely next month. If you want top of the line graphics you are going to have to replace your graphics card every 2-3 years.
Case in point: my desktop is about 4 years old and has a nVidia TNT Riva2 3D SomethingOrOther, it was not top of the line when I got it but was upper end. I would not even think of playing Far Cry or any of the newest mondo games on it, but it does wonderfuly on games such as Warcraft III (although I have played WCIII on my new 8600 and it looks WAY better). The mobo has a 2x AGP slot so my upgrade options are limited also. I am personally gonna wait for PCI Ex and then get a new system.
So back to the crystal ball and your question ... it is all speculation. We saw VESA slots for a while after ISA was introduced (or do I have those backwards?), ISA slots after PCI was introduced, PCI video cards after AGP was introduced (can still get them for that matter, but you have to look a little harder), etc. Yes I think PCIEX will replace AGP but your system is not going to go from screaming gaming system to doorstop overnight. It will take a few nights at least