Hi Slacker70 and welcome to the club,
YES, you do need a sub to fully experience the full impact of movies on DVDs or Blu-Rays.
For a good value sub, check the SVS PB12-NSD ($569) or the SVS PB10-NSD ($429).
Energy or Paradigm speakers = excellent speakers with very good value.
For your room size (17 x 20"), I will suggest the Onkyo TX-SR607 at only $350 right now, or the 606 at $320, or the 705 at $440, or the Marantz SR5002 at $350, or the SR6001 at also $350. All these receivers mentioned here are your best value on the market right now. They are well worth it over the 507.
#1 = Onkyo TX-SR705 or Marantz SR6001.
#2 = Onkyo TX-SR607 or Marantz SR5002.
* Just look at their own respective web sites for all the specs plus features, and also read some reviews (just Google them) to familiarize yourself with their various features and the ones that you can make better use of, in your own system.
The best deals can be found at "Accessories 4 Less" and at "6th Avenue".
So, for your three Energy speakers, a sub like the SVS PB10-NSD and a receiver like the Onkyo TX-SR607 or the Marantz SR6001; it will run you approximatively $1,050. And you'll be totally set up for the full impact in your 5.1-channel surround system (you already have a pair of speakers for your surrounds). Just don't dismiss that sub from the get go, it is one of the most important part of the puzzle. It is very well worth it, trust me.
And you'll be good for a very long time.
Start saving your money right now, so that sub won't be waiting too long. Because when you'll get it, you'll realise how right I was.
Cheers,
Bob