well with a case temp of 20 im getting 44 idle at 3500 rpm, i have a thermaltake xaser 2 case, chips a XP2700, and the other heat source is the viddy card, so its designed for getting the air from the front of the case, all the way round the cards and cpu and out the back at a high rate to stop ambient build up, sounds like you have bad ventilation or a dead air spot over the CPU. the case has 2 inlet fans mounted to the bottom front of the case, a side panel fan over the viddy card, 2 rear exaust fans, and the PSU has 2 fans, 1 to pull air in over the top of the CPU and the rear to act as a 3rd outlet.
bad ventilation will let some of the hot air being blown out the heatsink to come back in the top stopping cool air getting in.
another thing is i have to clean the fluff out the volcano 11+ once a month, that can take idle temps to 50 when its getting clogged
My friend's Barton 2700+ with a Volcano 11 at full blast runs about 42C at idle in a really really hot house, so I think 55 at idle is a bit high. Even under full load, his never approaches 50C. Perhaps your case is not exhausting the hot air quickly enough? Or perhaps the HSF is not seated properly... or as stated before, could be too much or too little thermal paste. Granted, my Athlon XP 2000+ ran at those temps all the time and I never noticed any ill effects from it.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS