Just how hot is 47C for a CPU?

Anything below 60 degC is considered safe. 50 - 60 is acceptable, below 50 is absolutely within the designed performance range.

I run my XP 2500+ overclocked to 3200+ and it runs about 48 - 50 at idle and 55 - 57 full bore and has been for almost 2 years now. Doesn't concern me a bit. It does require monthly (for smokers and/or pet owners) or quarterly (low particulate environment) cleaning to maintain these temps. The longer between cleaning, the more gunk builds up on the HSF and temps increase. A couple cans of air do the trick.