Chris, I'm not sure specifically about the Athlon chip you have, but I wouldn't worry about running it at 55C at all. I have an Athlon 1GHz (AXIA) and I'm currently running it with a single Delta fan (a very noisy, very powerful 7000rpm fan) slowed way down so it's whisper quiet plus one 80mm Zalman case fan from QuietPC and a QuietPC uber-quiet PSU (can't remember the model). The end result is very quiet, though hardly silent - just fine for movies though.
Anyway, with this setup the CPU runs very hot - around 70 degrees C most of the time (sometimes dipping down to 67-68 on a cool day with the window open...) and I have no problems whatsoever. I've had it running this way for 6 months now I believe, and not a hitch (touch wood). It's probably shortening the life of the chip significantly, but it's not like anybody keeps their CPU for more than a few years at best...
I've actually had this particular chip for three years now, and up until 6 months ago (when I converted it for quiet HTPC use rather than hardcore gaming) I had it overclocked from 1GHz to 1.333GHz (tho I was running the Delta fan at full revs, plus had 5 80mm case fans, so the thing sounded like a jet taking off...). So that's two and a half years at 33% overclock (CPU temp was never more than 50degC) followed by six months at 70degC, and haven't had the slightest hint of a problem with it. I mean, it's possible it's on its last legs and it'll keel over any second now, but I doubt it. AFAICT CPUs (or at least Athlons) are a lot tougher than they're made out to be.
Dunc