As you can see from the link, you'll barely be able to see any improvement on the image from a 50" 1080P picture at 9 feet away. You can save quite a bit of money by buying a 720P set and not notice any difference from 9 feet away (the kids sitting on the floor may notice a slight decrease in the resolution, but what do you care?). Personally I have the exact same set up (50" set at 9 feet), and I went with a 720P set for these reasons. When I went shopping I found a 720P set up right next to a 1080P of the same model, and I couldn't see any difference until I got to about 7 feet away.
1080p charted: Viewing distance to screen size - Engadget HD
Another way to save money would be to consider other technologies besides plasma. One of the selling points of plasma (besides how thin it is) is the very bright screen - and you just don't need that since you have good light control. Check out some of the rear projection sets or front projection.
Mostly will watch movies, TV (cable) and play the Wii.
