I'm running 43C max with my duron at 1gig, 2.2V - watercooled
Of course you don't have to turn to watercooling to drop those temps; there's a few things you need to look at:
1) The hsf itself
2) The thermal interface between the hsf and cpu core
3) Your case cooling.
A better hsf will definitely help your temps drop - a Globalwin FOP38, an Alpha PEP66 or PAL6035 will be great. It looks as though the best hsf out fo socket A at the moment is the swiftech MC462, though it's very pricey -
http://www.swiftnets.com/MC462.htm .
If you want to keep your standard hsf, take it off and clean whatever thermal goop is between it and the cpu. Scrape it off with a credit card or plastic knife if you need to (leave the cpu core alone of course), and wipe any excess off with a tissue. Lap the base of the hsf flat - keep sanding with progressively finer grades of sandpaper until you get a mirror finish (tape the sandpaper to a sheet of glass to keep it perfectly flat). Then get some arctic silver thermal paste, and apply it in a really thin layer between the hsf and cpu.
As for better case cooling, this is the most important part IMHO. Get at least 2 120mm fans, one as an intake at the bottom front, and another as an exhaust at the top above your cpu. Cut holes the size of the fans in the case so their airflow is unrestricted, and this should drop your temps down to low 50's.
Once you've done all this you should be seeing temps below 50C, hopefully close to 40-45C. 66C probably won't kill your cpu, but it's far from being a good thing
