The Core 2 Quad Q6600 is still a decent processor, and will be more than adequate for most tasks. Correct me if I'm wrong, but clock for clock, Core 2 Quads, regardless of the manufacturing process, is still slightly faster than the Phenom IIs clock for clock. At stock speeds, a Phenom II X4 940/945 beats the Q6600 at 2.4GHz. Overclocked, I'm supposing that the Q6600 will outperform the Phenom II slighlty (both at 3.4Ghz, for example).
The Core 2 Quad Q6600 will last you for a few more years, I'd give it around three years.