I have an Athlon XP system in an aluminium Coolermaster case (pretty much all components from overclockers - excellent site). The chip is the 1.8Ghz version sitting on an Abit board, under a Zalman Flower copper cooler, which not only cools extremely well but can be silent if the fan attachment is unused (depending on the layout of your case and power supply, the additional fan may not be necessary).
The chip is not overclocked and currently idles at about 38 - 39 degrees C, and even if pushed hard for a half hour or so won't exceed about 45.
Also, note that the cooling compound you use to mate chip to heatsink can have a large effect on cooling potential. I have used Arctic Silver, which seems to perform very well. Compound is probably the cheapest component in your system and returns considerable performance benefits, worth looking into!