Do you feel that the E6700 is limiting you to any degree? Whether or not you would notice a performance increase depends heavily on what you do with your PC. Perhaps you can use this site to get a general idea of what you can expect by upgrading as they allow you to compare the E6700 to the Q6600 with various benchmarks.
Personally, don't believe that you would see a significant enough performance increase by upgrading to a Q6600 that justifies spending an additional $250.
Many thanks for the reply.
The E6700 is not bad at all, the web site shows a slight improvement.
I may have to save up and go for a more powerful cpu !!