Do you really want to keep that much data on one harddrive?
Depends upon what you consider to be "data"? For myself, the primary application for a drive such as this is multimedia, which I don't necessarily consider to be critical data. Good news is that most of the newer drives have monitoring software/firmware which gives you warnings of any pending failures. My experience has been that a drive in a consumer system rarely dies without warning. They will usually start to act up, occasional boot failures, overheating, etc. I have had two drive failures over the last 3 years and both instances, I was able to transfer the data off the drive. If data integrity/reliability is a primary consideration, isolate important data into a single partition on a smaller drive. BTW, that's why they sell CD-RW.
Regarding two drives versus one, when dealing with video and audio files, having 200GB of storage available in a single drive solves a lot of problems. My media server is a work in progress that continues to evolve as better hardware comes out, primarily storage. I am migrating my entire audio CD collection (approx 1000 CD's, over half classical or jazz so the encoding req'ts are higher). After that is completed, then will start converting to digital, several hundred LP's. Because the server is in my family room, attached to my stereo and TV, one of the primary considerations is noise and heat. More drives, more noise, more heat, more fans...larger power supply, more heat, more fans, more noise, etc. By collapsing 3 or more drives to single unit, my system just became a lot easier to manage with more options where it can be placed. In fact, if video were not part of the equation, I would prefer 5400 RPM drives.
Bottomline, this is great stuff!