It will vary by plasma and there are different chips and programs they use for how the convert the content to their native resolution. Some handle it better than others, which is why SD television programming looks worse on some plasmas than others, even if HD looks great on both. I have both Panasonic and a Samsung plasmas at home, for example, and my SD programming and Netflix streaming looks better on the Samsung than the Panasonic. This just happens to be a strength of Samsung -- better display of lower than HD res content.
You are lucky with your Internet provider BTW. Where I live in Central NJ Comcast is worked up about people streaming video with Netflix or Hulu (because their POV is it uses a disproportionate amount of bandwidth and they aren't getting a piece of the revenue and it takes away from people paying for the TV service) so they setup the network to detect any streaming from those sites and they throttle the bandwidth down for them to make them almost unwatchable. So if I consistently average 7-9 mbps in general and can meter it before and after stream, the moment Netflix turns on it reduces to 1-2 mbps and the ping time increases to a crazy 5,000-10,000 ms. Since Netflix is forced to do the lowest resolution, it is only using a modest bandwidth so that doesn't account for it. The instant Netflix is turned off, my full bandwidth restores. And they started doing to a couple months ago -- it was fine until then. They suck. This is exactly why network neutrality matters.