640 GB, 7200 RPM, 32 MB cache, 5 year warranty: $75 ($0.12/GB), free shipping. It’s almost too good to be true.
Still, the best drive for MythTV is Western Digital’s low power series. I’ve been using the 750GB GP model with MythTV for about a year now and it has been silent and completely reliable the whole time. It contains an XFS filesystem, which probably helps quite a bit when recording 6 GB/hr (full HD resolution). The MySQL database that MythTV relies on happens to be on another drive, but I imagine it would be fine to put the database on the same drive.