WELl according to what u have written . u don't need a new heatsink. In fact the stock heatsink for core2 duo's are good and they have some room for overclocking s well as they core2's run cooler.
i guess if u are getting 45 deg then ur heatsink installation is correct.
im not clear about the room tempratures
Improve the airflow of the system by tying the cables and putting them to the side the drive bays. there should be minimum clutter over the motherboard to improve airflow
The power supply fan sucks air and throws it out not push it in so it is kind of an exhaust not an intake.
if u don't have a duct over ur cpu fan as shown in the photos of this case
http://www.silentpcreview.com/article210-page3.html
Then both the case fan and the power supply fan will pull the air out thus decreasing the airflow over the porocessor heatsink and thus incrasing its temprature.
Also the temprature of the processor cannot go below the room temprature.
about the lag during starting up of cpuz it's normal this is because cpuz reads information about ur computer everytime when it starts so it's normal i experience it my self . I also have a core 2 duo system.