Generally, the cooler the better. CPUs vary wildly in their operating temps. Some athlon thunderbird CPUs run at 50-60C as their normal operating temperature! Athlon XPs, properly cooled, run at 35-45C and Pentium IVs at 30-45C.
Overclocking is a risky business and should only be attempted if you know what you are doing and have adequte cooling, most CPUs could survive 70C but start to die after this!