I am looking at the LG dn898 (which for some reason is out of stock everywhere, but thats beside the point)

This dvd player will be hooked up to my ONKYO ht-r340 which decodes DD and DTS. Do I need to buy a dvd player with these decoders built in to take advantage of the DD and DTS? If not, would having a decoder in the dvd player and a dvd decoder in the reciever "mess up" to sound in any way?

Thanks in advance

DVD players send the DD and DTS to the receiver which in turn feeds out the appropriate surround sound format that you picked when watching the DVD movie.

Just connect your DVD player via optical or coax (RCA) cable and enjoy.