If you want to edit full HD 1080p video you want the most powerful system you can afford. You may get away with a less powerful system if all you want to do to your video is just join clips and produce. As you get more adventurous in your editing you will be grateful for the extra grunt.
Having said that your graphics card will also be an important part in producing your videos as PD has the option of using the power of your graphics card to speed up rendering/producing.
Now to completely confuse you I have made the decision to shoot and edit my videos in 1280X720 as the difference between 720 and 1080 is not enough to warrant the extra hassles and disc space. Maybe when I upgrade to Win7 64 bit I may go back to 1080 but not on my 32 bit system. Your requirements may vary.
Welcome to the exciting and frustrating world of video editing.
Cheers
Robert S
My youtube channel====> http://www.youtube.com/user/relate2?feature=mhsn