Yes, the Q6600 can overclock to 3.0Ghz with no issues.
At 3Ghz it will perform nearly the same as a stock E8400 in single threaded apps and MUCH faster in multi threaded apps.
Even at stock, you will have plenty of CPU power to drive your 4870.
With a stock cooler, you will probably be limited to 3.0Ghz.
With a good aftermarket cooler and a willing chip, you should be able to get 3.4-3.6Ghz out of a Q6600.