Well, I think I found the $250 solution to my HiDef problems. First, I did some research on the net, and present the results here. It turns out my MythTV upgrade is uPnP compatible. For you non-techies, what that means is, my backend system is plug-n-play compatible. It's a new standard for audio and video media. In other words, if you purchase a commercially available product that meets the uPnP standard, then it should work with other uPnP products, even my homebrew MythTV box.
After looking at the compatibility list, I decided the best option was the Buffalo Link Theater. I found the least expensive option on NewEgg, at $229. I've placed my order, and will report back with the results.
Goodbye Xbox. Hello Link Theater