If I were to use to additional speakers and place them behind me or to the side of me, would that be defeating the purpose of 5.1 or would it enhance it?

Basically, I would have one center channel and two mains in the front, one sub and two rears behind me and two more speakers to the side of me (8 speakers total).

Now, because my receiver is only 5.1 capable (DD/DTS), would I really not be able to utilize the extra rear channels - would they kind of wash out or what? What would my results be?

Should I dump the receiver and get a 6.1 or 7.1 receiver or just live with what I have? Could I go the cheaper route and just get an external decoder?

As you can see I am very confused as to how I should really be using my setup.

Thanks for any help.
My setup is below:

TV: Mits. WS65907
DVD: Mits. DD6000
HD/SAT: Mits. SRHD500
Receiver: SONY STRDA50ES
Amps: Parasound HCA855A & HCA2205A
Front sp.: (2) Energy 1.8
Rear sp.: (2) Energy C8
Rear Effect sp. (2) Energy C2
Center: Energy AC300
Sub: Energy XLS10
VCR: (2) who cares
CD: who cares

Just remember that the more speakers does not equal the better sound.

If you start filling your living room up with drivers all over the place, you'll find they start 'competing' with each other and actually deteriorate the sound rather than adding to the experience.

In your case, if you have extra speakers to use, put them in the side centers and run 7.1. There is little advantage in putting double sets at the rear for effect speakers.

That's just my opinion. Feel free to disagree anyone.

I would reccomend upgrading your reciever to a 6.1(one addtional rear) or 7.1(two additional rears) before doubling up speakers on each your right and left rear surrounds. Onkyo's 787, around $1000 has 6.1 built in(for 3 rears) and for another $300, you can get an additional 2 channel amp to run an extra 2 rear channels(so you can have 4 surround channels). I think you would be happier with this rather than just adding alot of duplicating speakers to your existing set-up. I would also say to use them up front rather than the rear if you want to stick with your sony.