DV uses 210 MB/minute (or 4.9 mins/gb). your drive after formatting will be about 56 GB. so you can get about 274 minutes of DV quality video on the drive (almost 5 hours). oh iMac. so this'll be the boot disk, eh? then subtract for the os, etc...still, three to four hours of video will fit on it.
but most editors don't use their startup drive as a media drive. get's too fragmenty in there, and the os and application accesses take away from the speed of the video's accesses...but, when i run out of space, i play of the boot dtive and don't drop frames on the DP500.
dependent on what you are making (style, effects, etc), after renders and exports, you'll probably find that you have enough space to comfortably complete one 60 minute project (more or less) before clearing it and starting another.