I have around 1800 miles on it (its the original chain and cassette) I didn't lube it for the first 800 or so miles, as I didn't know I was supposed to. I now try to lube it after every couple rides or so. So it sounds like probably just replace the chain this time, and unless it skips, keep the same cassette.

p.s. someone please tell me what 11-21, or 11-28 means when referencing cassettes. I've tried for the life of me to find out and it's driving me crazy.