52 is as high as I would let it go... not really going to damage the processor but it could make it unstable...
For heatsink reviews check this out... it's good for starters:
http://www.athlonmb.com/article-display.php?ArticleID=133
*make sure there is thermal compound/paste between the cpu and the heatsink (should be a very thin film, almost transparent)
*make sure the bottom of the heatsink is smooth... you can check it with a straight-edge. . if not smooth it (this is called lapping-do a search on google for heatsink lapping and i'm sure you'll find tons'of how to's)
*make sure the heatsink is seated properly
*use a quality heatsink (pure copper or copper based/aluminum fin are the best... all aluminum heatsinks require a high rpm (loud) fan to work well)
*make sure the case has good ventilation... Your power supply blows air out, the case fan probably does too, so you need to make sure the intakes in the front are clear of obstructions (tidy the cables up)
*make sure it's all clean (no dust farms)... dust is a good insulator (not wanted!)
If this is a new purchase, i'd take it back and have them do the above, and make sure they don't just set the temp threshold higher. 52 is already high in my opinion because this isn't an accurate measurement. . there is a diode under the cpu which is "supposed" to touch it but usually doesn't (and isn't greatly accurate anyway). AMD AthlonXP and future AMD processors contain on-board temp monitors, but the motherboard has to support it (right now I know of only 3 that do, 2 made by gigabyte and one by asus). After june 10 of this year any motherboard manufactured must incorporate this feature if it want's to carry the "amd approved" label. So, something to watch for if your out shopping...