Samsung UN46D6000 vs. Panasonic TC-P50ST30
First of all, sitting 12 feet from a 42-inch HDTV is way too far to be optimal; I think you'd be happier with a larger screen at that distance. LCD TVs with 120 or 240Hz refresh rate do sharpen motion, which is great for sports, but they also introduce a visible artifact called the "soap-opera effect" that makes images look like they were shot on video, though this affects movies much more than sports shot on video to begin with. Plasma TVs do not suffer from motion blur like LCDs do, so you might consider a plasma instead.
An Internet connection does allow the TV's firmware to be easily updated, including new apps to access online content. However, which apps are available is entirely up to the manufacturer and the deals it has made with content providers. I know of no TVs that offer access to content from twit.tv, but the Roku box does, so you might consider adding that to your system.
In your price range, I recommend the 50-inch Panasonic TC-50ST30 plasma ($1300, reviewed here, shown on the right above) and 46-inch, 120Hz Samsung UN46D6000 LED-LCD TV ($1300, reviewed here, shown on the left), both of which offer access to online content. The Panasonic is 3D-capable, while the Samsung is not. Also, the illumination in dark scenes on the Samsung can be uneven, a universal problem with LED-edgelit flat panels. Of these two, I prefer the Panasonic.
If you have an A/V question, please send it to firstname.lastname@example.org.