Depending upon how "golden" your ears are, I'd recommend you consider buying new front speakers to match your center speaker. Keeping the sound quality even across the front three speakers is highly desired, in order to avoid uneven tones as the action travels across the screen. I'd then continue to use the Polks in the rear. If I were unable to mount side speakers (on stands? from the ceiling as diesel suggested?) I would not try to set up a 7.1
Also, depending on whether you have even a partial side wall from the rear on your dining room side, I would not recommend dipoles even with a 5.1 setup. The main intent of rear dipoles is to diffuse (spread) the sound, in part by reflecting off the side walls, to help avoid localizing the surround effects. However, if you don't have a side wall on one side, dipoles may diffuse the sound so much that you don't even hear effects on that side, while simultaneously magnifying the effects reflecting off the other side wall and your window! I would use direct firing conventional speakers in the rear, about 6 feet or 2 meters above the floor, tilted inward and downward toward your primary listening position. This should eliminate both reflections from the window side, and loss of signal into the dining room.
