Conclusion: Core 2 Quad Q9550
If you don't want to settle for two cores, the low-priced Core 2 Quad Q6600 is a good choice and the currently cheapest available Intel quad-core. A Q9300 is a little faster and more energy efficient indeed, but also more expensive. The new middle-class quad-core Q8200 for about €180 has been available for a very short time only.
So it's better to take the "old” Q6700since it had to endure a big price-cut a few months ago. The Q9550 is recommendable as a high-end quad-core - the higher ranked quad versions are still too overpriced.