DV is roughly 5 minutes per GB. I don't know about encoding analog formats to the PC, but I imagine the space requirements would be a lot less. However, if you are using DV, then depending on how much video you have, 80 might suit you... with all the other stuff I had on my old HD, 80 wasn't nearly enough for me, and I quickly filled up the 60 GB external hard drive I had as well. On my new PC I have 240 GB, which gives me enough breathing room. Of course, YMMV.
"Film is often just business -- I understand that and it's not something I concern myself with. But if film aspires to be part of culture, it should do the things great literature, music and art do: elevate the spirit, help us understand ourselves and the world around us and give people the feeling they are not alone."