New HT
Sounds like your HT is off to a good start. Your current HDMI and digital audio connection should be adequate for the DVD player. For the digital cable, plan on another HDMI or a component video cable to TV and a digital coax or optical for the audio to the receiver.
Don't waste money on the Audioquest cables as there are plenty of lower priced, quality alternatives from bluejeans cables, impact acoustics, and others. Audioquest also subscribes to voodoo like batteries on cables.
A good 12" sub should work well in your room. Check out the online options from SVS, HSU, and AV123.com. If you'd rather stick with a local dealer than look for velodyne. Once you have the sub hooked up (with digital coax cable from the receiver) then add the center channel and surrounds next.