So was I the first time I installed both Coretemp and realTemp. then I noticed that the Tjunction was set differently in both programs. Changed the Tjunction setting in RealTemp, and they match perfectly now.
On a side note, I also have PC Probe II and that shows way different temps, but then, I remember this peice of software from my i955x P5-WD2 days with my Pentium 805D chip, and its been broken since then!