Every motherboard I've ever worked with does what I described.
Maybe they dont list it becasue they want people to buy the more expensive stuff? I bet it is 4200 binned ram anyway, even value ram would be able to handle a freq. increase that small.
OP: Buy the PC4300 if you like, for peace of mind, I am just saying that I'd be very surprised if normal speed ram didnt work. It will just bring the 4300 that is already in there down to 4200, which really will not be noticeable.