Yep a new PC would be a good idea. If partitioned the OS drive would not fragment, and depedning on how intensive you are with projects woul decie the size of drive needed for video projects, I use a 20gig partition for mine, which includes the use of two conversion progs for converting from avi (I am the only one in my family that has a DVD writer and so get the family cideos from family members in different countries to convert to DVD to spread through the rest of the family here) and TMPGEnc DVD author for converting the conversion into vobs. Saying that most of the projecs don't go above 4gigs as I only have a SL writer, so more space maybe needed if I go dual layer.
The trouble with adding a new drive, fritzi, is that some manufacturers can be strict on their warranties, so read it carefully blue or you may void it by adding hardware yourself. My friend was told by the company he got his from, I thnk it was Dell though can't be sure as I have built him one since and the old one is gone, that if he added a memory card himsef the warranty was void. Saying that this was a while ago and things may have changed, and I nor my friends have actually bought a system in such away as I tend to be called into build them.