Hardware MPEG decoders were necessary when processors were barely breaking the 400MHz mark. Now, they're largely unnecessary. Unless you plan on doing a lot in the background while watching movies, I wouldn't waste the money.
A lot of the functions you want are pretty well served in ShowShifter and its interface is perfect for televisions. I recommend checking that out.
Dscaler is actually what I use to watch TV in my room on my PC as I didn't like having a TV in here any more taking up yet more space. It's picture quality is unparalleled but it's not exactly TV friendly as far as the interface.
