Date: 10/6/2008 8:45 pm EDT
Unless there is some water damage the particle board should be fine.
Do me a favor and check the size and spacing of the floor joists,2x10 joists 16" on center are ideal. If possible confirm that the particle board is 3/4" thick. if not check to see if there is an additional layer of plywood under the particle board.
If every thing checks out, using 1/4" underlayment will be perfect, OSB will cost the least.
let me know about the framing. Also if you have questions about Quietwarmth I might be able to help. I just finished a few jobs with it and it worked out great.