Make sure that there are no obstructions in the vents on the case.
Believe it or not, one of the overheating problems on the Macbook was caused by a strip of protective film that was over the vents below the screen. Check there too.
I have an early 2008 Black MacBook with 4GB of RAM and the stock 160GB hard drive. Sometimes, when I have only Mail and Safari open and am just browsing emails, the fan starts to run full speed, and the internal temperature reaches 180