I have Denon receiver 3805 and consider buying Oppo BD 83. what's the best connection in term of audio i should use? Or should i have to buy a new receiver to take advantage of audio?

thanks.

With the 3805 I believe the only option you would have would be to use an optical cable. You would need a newer receiver with HDMI or enough analog outs to take advantage of the new sound formats.

The 3805 doesn't have HDMI connectivity, so you need analog cables to get the best sound.

On the back of the Denon there is a label named EXT IN. Plug one end of the cables there, and the other end to the Oppo's 7.1/5.1 audio out jacks.

The Oppo decodes True HD and DTS-HD internally, so you should be able to receive those advanced audio codecs.

Thanks.
I'm talking about listening SACD to take advantage of its pure sound format. I heard people talk about the bit stream and PCM. What's the difference? I guess i need to update receiver with HDMI in as well.

PCM is used when the player decodes it and sends it to the receiver. Bitstream is used with receivers that are able to decode the formats.