I have a Harmon Kardon system, it's not 5.1 or DTS and I would like to upgrade, my question is, do i need to purchase new speakers and a sub too? or can i just upgrade the receiver?
My DVD player is DTS (if that makes a difference)

Advise please!

Well, to actually get 5.1 sound, you will need 5 speakers and a sub.

Let me clarify a bit - I already have the speakers and sub, I have an older surround sound system, I just wanted to know if I can upgrade just the receiver..

In that case, yes, you can just update the receiver.