Hi msurge,
I think you would probably be better off with a mattress protector than a mattress pad (which has some form of padding and will have a much bigger effect on how the mattress feels).
Post #89 here has more about the different types of protectors and some of the tradeoffs between them.
The most common choice would usually be one of the membrane types of protectors which will generally have little effect on the feel of the mattress although the membrane is not as breathable as some of the other types of protectors so for a few people it can have an effect on the sleeping temperature of the mattress.
Phoenix