unless there is a very clear case of bottlenecking (e.g. well under 90% utilisation of the GPU and crap frame rates at actual gaming settings), I would always advocate buying a new GPU over buying a complete system rebuild - if it gets you the results you want then you've saved several hundred pounds, and if not you can always DSR the card and then start saving for a rebuild
Then I guess you and I see bottleneck differently. Even if a GTX580 dropped down to..say 92% GPU usage, that would already mean it is operation at slower speed than a GTX570/GTX480 or at around that level. If that is not a CPU bottleneck, I don't know what is.
After a bit more thinking, it might be not a bad idea to go from crossfire 4890 to a 7850, but not because of the reason GPU grunt of two 4890 in crossfire is lacking, but more about the poor crossfire performance and support, which could make the crossfire 4890 not performancing as the hardware are fully capable of.
And I'm not gonna bother to argue with you about CPU bottleneck anymore...as I said before, just because the games YOU play the CPU doesn't (seem to) bottleneck the graphic card, doesn't mean there are absolutely
no games at all out that the Core2Quad/Phenom II X4 doesn't use the graphic card fully. You had your experience of Q6600 doing well with GTX580 in games YOU play, while I had mine of my Q6600 overclocked to 3.6GHz bottlenecking my 5850 (which is slower than the GTX580 by a big margin) down to 60% GPU usage and frame rate drop all the way down to low 20s (also reported by the beloved MSI Afterburner) in online games which I played, and in SC2 during intensive moments, GPU usage drop all the way down to 40% and frame rate down to sub-20fps.
While during standard gameplay for most games an overclocked Q6600 wouldn't do too bad and would hold 60fps with a high-end card during not intensive moments, but the biggest problem with it is it doesn't have what it got (may it be CPU grunt as well the architecture design itself with lower minimum frame rate- even if comparing to the E8000 CPUs) to hold the high GPU usage and frame rate during those suddenly moments which the scenes get very intensive, which result in sudden huge frame rate dip within short period of time, making drop in smoothness very noticable. If you say you have NEVER seen sudden huge dips like that with a GTX580 on a Q6600 in ANY game, then it makes me wonder if you are only arguing for winning argument's sake and not interested in the fact. And honestly...if you ask anyone about an overclocked Q6600 with a GTX580, most of them WILL tell you the overclocked Q6600 will be holding the GTX580 back, not like the overclocked Q6600 won't bottleneck the GTX580
at all that you claim.