About my C2D E6420 temp..

Hi ...

Sorry if this was asked previously..I have a system
PSU 600Watt
Gigabyte 945P-S3 rev 3.3
Intel C2D E6420 (stock speed...I never overclock)
Ram Patriot 2GB (1GB x 2) 667 MHz
Hard Maxtor SATAII 7200RPM 160GB
VGA Nvidia 7900GT 256MB
Voltages and powers and fans are normal..
and other usual components

My room temp. is about 25C...My idle CPU temp. is 38C...messured by Everest (AS CPU temp.) with 28C and 29C for core #1 and Core#2 ...After an extensive application like Call of Duty 4 game of Virus scan ...It reaches about 48 or 52 C or something like this and the cores about 43C and 45C...I didn't apply a thermal paste when I installed the CPU (as the buyer told me that the stock boxed fan contains already some on its lower side)...

Another question is that I heard that the CPU temp.(overall) should be less than the temp of each core alone...But in my case I get the contrary...like 42 and 28,29.....What is the probelm ?

Now do you advice me to reapply a thermal grease and reinstall the stock heatsink or everything is all right ??

Thanks in advance...