Hi Brent,
There are a few things you can try which may fix your quality issues...
First, I would try different codecs. The nvidia seems to have the highest success rate for the majority of people so if you're not using it...give it a try
Next, depending on your TV type (Plasma/LCD/CRT), you can try changing the Display Device type in the MCE settings. MCE will adjust some display settings like resolution to best match the selected device. I think they do this to try and give the best quality on average accross a wide range of display types. While this is probably a good idea in general, many have reported quality issues including myself.
MCE seems to soften the TV image making it look dull, so a little tweaking can potentialy gain some significant results...I know it did for me
Another thing is to make sure the latest drivers are installed for tuners and sound hardware to ensure MCE compatability.
Cheers