I was looking at the Q6600 yesterday on the egg. $269 w/ free shipping, and it is a drop in upgrade. Right now I have a E6400 running 3.2Ghz
Also worth mentioning the Q6600 SLACR or 'G0' stepping is incredibly overclockable.
Running at 95W compared to the 130W 'B3' steppings. Or the ludicrous wattage demands of the current AMD lineup, makes this processer signifigantly cooler both at native speeds or with an overclock. Even with a minor increase in the vcore it is still an energy efficient CPU.
The standard 2.4 Ghz is pushed to 3.2Ghz (33% increase) with little effort and little extra heat output. The current range of air cooling HSFs adequatly cool the extra heat produced by 4 cores with no problems whatsoever. Although if overclocking I'd still go for a Mugen or Thermalright Ultra etc as opposed to the standard retail HSF, However 3.6Ghz has been reached on a standard retail HSF
True, the 45's are right round the corner, and I think everyone wants AMD to have a better year than they've suffered previously.
But as the market stands currently, the Q6600 is the best bang-for-buck by a mile.
As more developers are embracing multi-core utilisation in their code, it seems multi-core processor arrangements are going to be the way towards higher overall speeds.