And a large part of what makes a better GPU is RAM bandwidth. If you're using integrated graphics there actually tends to be a quite large difference between using system RAM clocked at 1066 and RAM clocked at 1866. If you're using a discrete GPU card the RAM that makes a difference to your gaming performance is already soldered onto the card, but that might still see an improvement from faster memory technologies since the people who make those cards could use it.
If you're using integrated graphics there isn't any PCI bus involved. Even when graphics was off-die it was on the Southbridge.
And the bandwidth between the GPU and the GDDR on the graphics card doesn't have anything to do with the PCI bus either, except to a small extent when synchronizing with the CPU or initially loading textures or whatever.
Apart from the cutting-edge APUs (like AMD Kaveri implementing hUMA), the memory of integrated GPUs and previous generation APUs was separate and copying chunks of it in between happened via the bus.
Plus since we're talking about cutting-edge gaming, integrated graphics is irrelevant (and so are APUs).
