LinuxMCE needs at a minimum 30 to 40G of HD space. The OS needs at least that. And then there are the MD images which require at least 10G more or less. I use a 500G drive as my OS drive. Also, if you intend on using MythTV, it also uses the OS drive to record media, at an average of 2.1G/hr for SD content captured on say a pvr 150/250/350/500 card. So ideally for multiple MD's, I have 5, You want at least 250G, add a decent amount of recordings to that, and that is why I have a 500G drive for this purpose. I have a 1TB, 250G, and 80G drive for everything else, DVD rips, downloaded movies, music, etc.
So keep that in mind for your installation.
Welcome aboard, and Best Regards,