The core temp(s) are what you should be looking at. CPU sensors can be either on the socket, or hooked to the heat spreader, and generally aren't what you should be looking at
I normally watch the core temps. I believe that is the temp that the processor itself it at. I have a Core 2 Duo E8400. With the stock fan and heatsink I was running at 73C, which is way too hot. I replaced the heatsink and fan with one from newegg and I am now running at 53C, I actually just did this last night. I have read that 45C is what people seem to like to shoot for and I beleive that 53 is ok but not great.