Microstuttering tests on ATI Radeon HD 4870 X2

[gosh]

CPU bottleneck somehow "synchronizes" frames, because GPUs have to wait for next frame.
Wors scenario of MS is in CoJ, which is completly unplayable below 70 FPS.
GRID is completly unplayable below 40 FPS and at 50 FPS you can see it starts to slow down.
Crysis is quite OK at 40 FPS with CF because of motion blur.

One alternative explanation of why the game could be smoother if the processor works harder could be how games handle threading.
I think that most games that are using threads have one render-thread. This thread is probably the most important also, it may have higher priority or it could even be that the game reserves one core for this thread. If the game prioritize one thread to one core like this then the game has more control over how the game behaves on the processor and how the GPU is fed with data for rendering. There is some drawbacks using this technique, it scales well to two cores but more cores (threads) that are used and it will not use the power of the processor if the processor has more than two cores.
If the game doesn’t take control of how the scheduling of the threads are done and how they are located. Then this task is handled by the operating system. The operating system will place threads for rules it has. Maybe it sees one core that isn’t working and another core has two threads, then it might decide to move one thread to that core. I don’t know the logics on how the thread scheduler works. I think that vista has a much improved scheduler compared to XP though. It could also be that it places two render threads on the same core and if one core will handle two threads it will of course slow down the game.
If you are running one C2Q and one thread is moved from one C2D to the other C2D (C2Q is two C2D glued together and the cache for each C2D doesn’t have any communication). The cache data is invalidated and the core needs to get data from memory, this will stress the FSB and the computer slows down for a fraction of a second.
It could be that if the processor doesn’t work as hard and it is a quad, then threads are moved around more than if the processor is working harder.

This is just a speculation.