The Q6600 was a pretty decent chip in its day, but many modern games will be held back by the low clock speed. Having said that, you will definitely notice an improvement with a newer GPU. I would start thinking about a replacement CPU/mobo as well though, or at least overclock the 6600 a bit.
If you are only looking at getting a 7850 at best then I see no reason to upgrade the CPU. Overclocking to 3GHz would help but there is no need to it will not hold back a 7850 except in the most CPU hungry games.