CPU bottlenecks

CPUs can allegedly bottleneck a system significantly.
I can understand why it's not worth shelling out on a £250 graphics card on a a 1.2GHz single core CPU, simply because games wont be able to process enough information to run the AI etc even though the graphics could otherwise run smoothly.

But I've heard things like "A q6600 (quad 2.4GHz) is a bottleneck for a 4870x2". To me this sounds ludicrous:
Playing even the most demanding games, a Q6600 will still only use at most maybe 25% of it's TOTAL processing power (please prove that statment wrong :) ) and assuming games are happy to run on quad core (I can understand if games run off a single core at 100% and leave 3 idling), this seems reasonable.

Now, if I put a 4870x2 for example next to a Q6600 and ran Call Of Duty 4 on full, my CPU would STILL only use maybe 40% of it's total available processing power MAX - a modest increase - this is because, I understand, my CPU has to feed data to my GPU, which with a faster or stronger GPU does require a modest few % more CPU usage.

Basically, my question is:
Is the ONLY way that a CPU might bottleneck a graphics card be by the CPU lacking enough processing power to actually feed info and work the AI etc etc?

Can a CPU that's only ever under 40% load in the most extreme gaming situations actually be bottlenecking a graphics card by (presumably the only way it can) not feeding it enough info?

Hopefully I'm being clear, if not:
My CPU is handling all the data that's fed to it instantly, passing relevant data on through the mobo to my graphics card without any sort of lag - my gfx card isn't left hanging for data because my CPU is lagging due to lack of CPU power.

Is that pretty much a straight guarantee that the CPU SPEED (I know there are other aspects like cache, FSB and transfer rates to think about) is not holding back my system?

Sorry for the long winded explanation and thanks for all your help
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  1. The reason your Q6600 isn't using 100% of its power while gaming is because the game isn't threaded to use all 4 cores to their full potential, so even though you only have 40% cpu usage, it is possible that it can still bottleneck.
  2. Thanks.
    This is actually a hypothetical question. The game in question could easily be doing 40% on one core, it shouldn't matter regarding the bottlenecking question?
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