I use XP MCE for everything but watching and recording TV. (Actually got it because it looked cool and was cheaper than Pro
) One thing I do use it for is Video editing, and boy do you need space. My 320 Gig is already 1/2 full with video, which is about 1 GB every 4 minutes uncompressed. I am thinking of buying a 500 gig when I need it which won't be too long. This is all regular 480p footage from a sony miniDV camcorder.
As for my answer-go with at least 500 GB, especially if you will be working with HDTV.