Realtemp is more accurate than coretemp with 45nm chips. (again because it guesstimates a more accurate TJMAX)
For a GO Q6600, use Coretemp.
The AC7 should provide slightly better numbers than the stock cooler
On a quad core, the temps will vary between cores based on load and also contact with the HS. I had a thermalright ultra 120E and a Q6600 GO (the TRUE is the best currently) and my temps were about 4-8C apart on cores. Once I lapped it, the variance dropped to 1-3C between cores.