What's wrong with my cat?

Calling all fellow cat lovers! I am really worried about my cat and I have spent the whole day stewing about this and searching the internet. We took this cat in when she was about a year old, after someone dumped her at the side of the road. She's like family. I plan to make an appointment to take her to the vet later this week (have to wait for payday!). She is a 13 year old tortie who has had no medical probems until she suffered two bladder infections over the past year: one in September 2009 and one in January 2010. We think the bladder infections were because of stress when we had another cat staying with us temporarily. Her weight has always been normal, about 7 or 8 lbs. I have noticed over the last few weeks, she is peeing and drinking more than normal. She's still eating a lot and playing normally. I have read a lot about the various causes, primarily renal (kidney) failure and diabetes. I am hoping (praying) it's diabetes kidney failure. What do you think the chances are that it's something less serious?