Comcast passes down the signals from the networks as is. Local stations, networks and cable channels only broadcast in 1080I and 720P. No one broadcasts in 1080P. Switching to a RNG200 will not get you any better picture or higher resolution than you are getting now. It is the same with all TV and Sat providers.
Since your 8300 is a HDMI box, there is no advantage to a RNG200 unless you have a 3D set. Keep checking back at your local office, since there is a rush for HD boxes at the holidays, you might find better supply of the new boxes around February if you really feel you need one. You won't see a difference in picture quality or resolution.
Most 1080P TV's upconvert all the inputs to 1080P which is the native resolution of the display.
Thats interesting. Because I called Comcast and asked for a more updated box and they told me that the 8300HDC is the most updated that they have. I found this to be disturbing as this box came out in 2004. How do I go about getting the most up to date one? I have a 60 inch 1080P TV and only being able to get 1080i is almost a dealbreaker