What's a good temperature Celcius to keep a CPU around?

It really depends on the CPU. I'd keep it below 60 degrees Celsius.

what CPU are you running? if it is on a laptop some chips can handle up to 100, the colder you can make it the better

I like my 3.6 60C or lower and older 2.5s @ 45C and under...

If your cpu under load gets anywhere near 60C, then you need to do something. Depending on your cpu, it will start to stutter. A safe range is around 40-50 at maybe at 55 if its been under load for a long time like 3 hours. If it's 50 at idle you need to do something. I have something better instead of a stock heatsink and fan. My temperature is at around 45

-273.15 Celcius (0 Kelvin). Overclock it as much as you like, then ;)

Ok so, Intel says that the max temp (for your CPU) is 73.3c.
So that means anything bellow 65c is good for full load,
and anything around or bellow 40c idle is also good.

Hope this helped.