Prime is good program but also use OCCT test too sometimes prime will pass and occt won't and vice versa. I would suggest running OCCT for an hour then let prime 95 go overnight. As far as your q6600 standing up to newer CPUS it dosent it is a 65nm CPU which was good when it came out but like all CPUS that dont last long compared to an I5 the I5 will smash it.
I have read the guide. I have successfully overcloked the Q6600 by increasing the bus frequency to 400MHz so the chip runs at 3.2GHz. RAM runs at 800MHz. I did not modify any voltages except the RAM which I put up to 2.1V. I am not sure what the CPU and other voltages are, perhaps the motherboard does it automatically, perhaps they are running at stock values. Chip runs cool at about 35C with no load and 50C with full load. I ran Prime95 for a couple of hours and it was fine.
I have also overclocked the i5 750, I raised the BCLK to 160MHz from 133MHz, running the RAM at 1600MHz (10:1) and using the RAM's own SPD profile, I believe voltage was raised a notch, but all automatically. CPU multiplier set to 21, so I am getting 3.35GHz clock on the CPU. Chip runs at about 30C with no load and 50C with full load, again I did not go anywhere near any voltages.