I've put together a backend MythTV server on an older IBM ThinkCentre (P4, 3Ghz) that I picked up second hand for $60. The tuner is a dual tuner HDHomeRun. I'm able to record 2 OTA HD streams simultaneously on the ThinkCentre.
I can then stream these recordings to Western Digital Media Player Plus which can play the recordings seamlessly but often isn't open to shuttling forward or reverse for some recordings.
The cheapest fully functioning frontend stutter-free playback I've set up is a Windows XP on another P4 using (not VLC or Windows Media Player, neither of which work) a very nifty program called "Media Player Classic" (not affiliated with Microsoft).
Since this set up was less than elegant, I sprung for upgraded motherboard with a newer AMD Dual Core processor and now use XBMC installed in Ubuntu as my frontend. With the right hardware, XBMC makes a terrific frontend but don't waste your time trying to get it to stream HD files to a 5 year old computer. There's no reason it shouldn't be able to handle that, "Media Player Classic" can, but unfortunately XBMC is more resource hungry.