According to the RIAA, which seems to have become quite powerful as of late, you can pay about 15K per song, or some bullsh*t like that. I do wonder though, if some prankster is behind a lot of these messages.
WRITE Comcast before doing anything else. Calls are difficult to maintain legal documents of. WRITE to Comcast, using certified mail so you have records of sending the document, and have its sending affirmed by a notary official or two. Also have him/her verify its context.
Ask Comcast ONLY if "they sent you a warning about file-sharing, and about sharing copyrighted files."
Do not ADMIT to sharing them at all. Do not say "the files I was sharing."
By doing these things, you can A) verify it was actually them, and B) avoid being made an ass of in court for admitting you share copyrighted materials.