They won't take you to court for $550, I am assuming that is why they are going through the collection route. Do you have a copy of any paperwork for when you cancelled your gym membership? Did he gym send you any notices of payment? I would request this from the collection agency.
It sort of sucks that this small sum can affect your credit rating. It has mine, I owed 750 and they are trying to collect from 5 1/2 years ago. Oh ya, you heard it right. I only recently discovered it has been on my credit report since 2004 and it affected me buying a car in 2004 (I assumed because I had just started a new job), and almost affected me getting my Federal Government job (luckily my supervisor thinks the Finance departments at Universities at invalids) so I still go the job, but had it not been her I probablyl wouldn't have gotten my job.
I suggest checking your credit report first and going from there.

