PCI Express, Continued
The reason why there isn't more of a performance payoff is simple. Games are designed in such a way that they don't send more data to the graphics card than the AGP bus can handle. Otherwise, there would be noticeable stuttering in games. On top of that, the amount of memory found on modern graphics cards (up to 256 MB) makes it completely unnecessary to page out data such as textures to the system memory via AGP. In all likelihood, this won't change with PCI Express either. After all, the 4 GB/s it offers seem positively tiny compared to the 30 GB/s of bandwidth that today's top-of-the-line cards can support internally. In short, swapping out data to system memory, either by AGP or PCI Express, simply doesn't make a lot of sense.
If you twiddle with the performance-graph feature implemented in NVIDIAs latest non-WHQL driver v55.56 you can see Nascar Thunder 2004 as one of the most texture and geometry intensive games today. But even this game does not address more than 64 MB of memory in 1600x1200. We also ran AGP 4x / 8x tests in this game - we found no differences!
So why do we need PCI Express for graphics? Well, for one thing, it's new, and, as the PR departments of all of our favorite companies never tire of telling us: newer equals better. Prepare to hear this a lot in the future - it will probably be the main advertising strategy for this new technology. That's not to say that PCI Express cards won't be faster than their AGP counterparts - they probably will be. The question is how big this margin will turn out to be.
Based on our AGP measurements above, which are admittedly hard to apply to this scenario, we shouldn't expect to see dramatic improvements - "incremental" would probably be the better adjective here. Whether and to what extent this will change in the future largely depends on how quickly PCI Express gains market (and consumer) acceptance. As always, when there is more bandwidth available, developers will find a way to put it to good use, driving the technology into the mainstream. Which brings us back to the future perspective.