DVB support in LinuxMCE is something that can be done now, but not unless you manually set it up.
I can not help you in getting HD working on DVB-C, but I can show you the DVB-C card I am using and have working in LinuxMCE. For now, all I have working is FTA, and no HD (They are not FTA).http://tven.terratec.net/modules.php?op=modload&name=News&file=article&sid=267http://www.linuxtv.org/wiki/index.php/TerraTehttp://tven.terratec.net/modules.php?op=modload&name=News&file=article&sid=267c_Cinergy_C_DVB-C
Not sure if you can find something this new at ebay, but any HD capable DVB card is new...
It is not easy to install, and it is even more difficult to configure in linuxmce...
I reccomend waiting for 0710 when VDR is "complete"
It you get your card you can try this WIKI for tips:http://www.linuxtv.org/wiki/index.php/TerraTec_Cinergy_C_DVB-C
I believe all DVB cards that are Linux compatible will work in LinuxMCE. But to configure them in MythTV requires a lot of work and I have not been able to find a wiki that takes me all the way. Only bits and pieces. VDR not that hard in 0710b3 and it does work great... The problem is that the VDR UI is still in beta and is not complete.