Yes, you would be correct.
A stock Q6600 with RAM at a 1:1 ratio to the FSB will be using DDR2 533 speeds.
I would not recommend purchasing DDR2 533 for your system, however.
Stick with DDR2 800, 4-4-4-X.
If you want to run it at a 1:1 ratio, you can drop the timings way down.
However you do it, you will not see more than a 1-2% performance drop/gain.
Here is how it works.
DDR2 RAM is running double data rate to the FSB clock.
With a Q6600, you are using a 266Mhz bus stock or 1066Mhz Quad Data Rate FSB.
At that FSB, you would either double the base frequency or half the rated QDR frequency to obtain your 1:1 ratio RAM speeds.