What are your main uses for your computer? If your gaming and you cant overclock that to 3ghz it will not be that much better then what you have dont get me wrong it will be faster but I dont know if its worth the money. But that all depends on you uses for the computer.
If you can find it for $100 or a little less would be better but I wouldn't spend more then $100 for it because I dont think you will even be able to overclock on that board and it is kind of a slow Quad at stock speeds.
For $150 you would be much better getting A cheap AMD CPU and Motherboard.
You can find used Q6600s going for around $100-$120 on ebay. There are also people here who probably have Q6600s to sell.
If you can OC on your mobo, the Q6600 will be your cheapest and best option for a good upgrade. Clock per clock, the 65nm Core2Quads perform about equal to the phenomIIs...so overclocking it to 3.0GHz or over will give you a very good performing quad cpu.
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.