I just finished building my first pc an on first boot the core temps are showing as 51, 48, 39, 38. I applied thermal grease and the heatsink that came with it. Are these temps okay for a first boot? I was expecting them to be a little lower.
You really haven't given enough information to truly advise where your temps should be.
The type of thermal grease can affect ultimate temps, some of them like Arctic Silver 5 need time to set up.
Also, the airflow through your case, and the heatsink on your CPU make a huge difference.
If you're using the STOCK Intel heatsink that came with the CPU, I'd say you're normal.
The 51,48 numbers suggest that Core 0 and Core 1 are being used. The 39,38 temps suggest Core 2 and Core 3 are sitting at idle. With 39/38 as idle temps I'd say you're doing okay if you're using a stock heat sink. Good aftermarket heatsinks should get you lower than that with stock speed/voltage settings on the Q6600.