I would think anything 55c or lower is ok for an AMD.
However, with my experience, I've found that the cooler your cpu, the more stable your system is, even if the cpu is in "acceptable" ranges.
I had an overclocked p3 for a long time and if the temp was 45c or lower it was alway rock stable. But if it got 50c or higher it would crash sometimes during 3d games. Heat is the enemy and if your a power user that maxes your system out, its best to keep things cool as possible. I would say 55c load is a bit high, I'd shoot for 50c MAX temp.
I have an AMD XP 2000+ at 1.75ghz (140fsb). I'm using a Vantec solid copper heat sink i got at Comp Usa for 35bucks and some Arctic Silver3 (which is overrated IMHO, didnt see any improvement using AS-3 over silicon grease). The heat sink has a 7000rpm fan on it.
My temps are 38-39 idle and 43c under MAX load. Only Prime95 will get me up to 43c. Ambient temp plays a big part since i keep my house 68-70deg F.
I have an Antec Server case with 3 case fans which helps too. I would say keep your heat sink but lap it smoothe on bottom. Just get a higher rpm fan and apply a THIN layer of thermal grease. This will drop temps alot. Its not neccessary to get a fancy 60dollar heat sink. I've turned stock OEM coolers into very good coolers with lapping/thermal grease and better fans.
good luck