Don't worry about the PCI bus limiting you as much anymore. Remember the bus was the limiting factor when the video cards only had 4 megs of RAM, not 64 or 128 that is standard now. This is also the reason why there was no performance increase in AGP 6x to 8x, cause the cards are not using the bandwidth. I would be willing to bet that you will not see much of an increase from AGP 2x - 6x either. Not that I know of anyone doing a test on this.
Hey Anand you think you can do a performance test for modern video cards with 64meg or more video RAM for the benifits of using AGP 1x vs 2x vs 4x vs 6x vs 8x, and possibly PCI?
That would be the true way to find out, but I suspect that the performance hit would not be as high as you think for a card with 128 megs video RAM.