I've been there too, in May 2003. I had a g/f from Pak Chong and we did the trip to Khao Yai with most of her family
It's a beautiful place with fantastic views and some good waterfalls. I'll post a few pictures later when the current g/f is watching her Thai soaps.
Although the family lived in Pak Chong, it was quite a way down a dirt road - about 2 or 3 km and the house was not very "farang friendly", so we - she, her 6 year old daughter and me - stayed in a Hotel in Pak Chong. Very big hotel just before the town centre on the left, reasonable rates, but can't remember what the name was. It used to get full at weekends.
Edit:
It was the Pak Chong Landmark.