Well first, the 7800GS is available in AGP. As far as the rest, depends. I'm not necessarily limited to 2x/4x cards. My slot is AGP 2.0 and provides only 1.5V. However, all versions of AGP must be compatible with the versions before it. Therefore, AGP 4x it must be compatible with 4x, 2x, and 1x. Kicker is though that 1x and most 2x cards signal at 3.3V which makes them incompatible with my slot.
AGP 8x cards are signaled at a voltage of 0.8V. But like before all versions of AGP have to be compatible with the versions before it which means 8x cards must be compatible with 8x, 4x, 2x, and 1x. Now it depends upon whether a particular AGP 8x card can be signaled at 1.5V. If it can I'm good to go, if it can't it may still run but might be unstable.
Now the good news. All newer nVidia cards are fully AGP 4x/8x compatible which means they are universally compatible with either 1.5V or 0.8V and will adjust themselves automatically. So back to my first question, between the 7800GS, 6800GS, and 6600GT which would be the most suitable for use in my computer (which would not exceed the bandwidth I can provide)?