There are more factors at play than just the clock rate (GHz).
Since the Q9550 is in the Q9*** series, it has 12mb of cache while the Q6*** has 8mb, which gives the Q9550 a performance edge at the same clock rate.
It is also worth considering the Q9***'s are made on a 45nm process which means they run cooler and OC better.
In general, there are a number of factors that contribute to the performance of a processor other than just clock rate, such as cache, fsb, branch prediction, pipelining, # of cores ...
The only time you should compare processors based purely on clock rate is if all other factors are the same, like comparing a q6600 vs q6700.
The best way to compare processors is to check out their benchmarks.