The advice about having a 2nd hard drive to download your video is good. I use Pinnacle Studio 8 as an editing tool and the transfer of video to the pc via a firewire card is easy.
The amount of video that you download to the pc , will depend on the quality of your original shots. If you are just shooting willy nilly without any forethought or planning, then you may find; depending on how fussy you are, that you only have a small amonut of video worth downloading and editing. In my first efforts with a holiday video, I was lucky to get 10 mins out of 3 hrs, but thats down to crap camerawork.
The editing packages are so good these days that you will find that you want to shoot good quality video in the first place and this to my mind is the key.
If you are wanting to add music etc then again this will take up more hard disc space. For best results, always edit the video around the music, not the other way around. The picture editing is very easy with a good package, it's the sound that is more difficult.
Hope this helps