What do I add to make my system HT ?

If I'm getting this right, then if I was to purchase a Nad T773,arcam avr300 or dennon 3805 any of these would be all I would need not counting center speaker and rears? Any preference on which of the 3 would serve me best with my particular system?

Arcam AVR300 by leagues over the others mentioned. Great product. The AVR200 is an even better deal at about 600 used if you have speakers that will work with 70 watts (five channels only; not 7.1). I owned one for a bit and sold it as I wanted the 7.1 capability. I miss the Arcam and should have kept it as I will probably never move to 7.1. The wife is less than supportive of additional speakers.

The cheap solution is "Looping in" a receiver into an "auxilary or bypass/tape/monitor input", which would power the rest of your speakers, and add processing.
HOWEVER, for simple improved movie sound from your existing system, I'd simply add a Digital process to your mix, looping that in instead....thus keeping the system 2 channel. You can then add a necessary sub, and get all the processing you NEED for DD/DTS. You can't get the same dynamic sound simply processing in a DVD player, etc...it's not as good...and you need a processor of some sort for movies via DD/DTS.