New system hard drive advice needed

I am building a new system but I am up in the air as for as CPU MB combo. I am currently putting together all the other hardware. What I plan to do ultimately is run 3 hard drives actively, one with OS and all installed programs, one with all data, and one to back up the data files to. I also would like to make a copy of the OS/Programs disk leave it in the case but not connected until a problem is encountered that might require a clean OS system. I then can run virus scan on all other discs to clean them. I am going to run Windows XP for now and plan to go to Win7 once a service pack is out. Does this stratedy sound good? Also what disksizes should I go with? I was planning on 4 500 Gig of the same brand and type for simplicity sake, but 1 TB are getting cheaper eventhough I don't think I would ever use 2 TBs between the OS, all programs, and data. Would there be any advantage to smaller drives like 320's for the OS/Program disks?

Thanks for any assistance.