First of all, your receiver does not need to support HDMI to use it with your new TV.
Simply connect via digital audio (s/pdif) from either the TV (if you use antenna), the DVD player, or your set-top box, and you will have surround sound.
Regarding the comparison; we need more info.
Is 850 W or 550 W peak or RMS?
How much is per channel in the two systems?
First of all, your receiver does not need to support HDMI to use it with your new TV.
Simply connect via digital audio (s/pdif) from either the TV (if you use antenna), the DVD player, or your set-top box, and you will have surround sound.
Regarding the comparison; we need more info.
Is 850 W or 550 W peak or RMS?
How much is per channel in the two systems?