Once you get that 50" G10 plasma, I'd suggest getting at least a DVD player (if you don't have one) or a blu-ray player if you can. Your current receiver can handle Dolby Digital and DTS, and those will blow away Pro-Logic. I don't know if you've ever used those, given your other thread on connecting your VCR and your talk now about Pro-Logic. VCR tapes won't have those audio tracks, but DVDs and blu-rays typically will.
A DVD player or blu-ray player will have digital audio outputs that can connect to your current receiver, as well as to any modern receiver.