Unless you are working with uncompressed streams, and even they'd have to be HD, you don't need anything faster than one of the current 7200rpm drives available today.
DV data rate is 3.6MB/sec, well below most any current drive, which can sustain over 20MB/sec. Even capturing lossless compression using huffyuv is only about 10MB/sec, which I was doing 5 years ago on ATA drives with no problem using a PIII850.
The new HDV format is also 3.6MB/sec so no worries there either, and if you edit it using the Cineform CFHD intermediary codec it's still only 10MB/sec.
Just make sure it's a 7200RPM drive placed on a secondary channel and big, video projects will tend to eat up lots of disk space, even small ones. Cache size won't have a big impact because the video files are so large.
I recently co-wrote a book on HDV called "HDV: What You NEED To Know" (on Amazon now). Have written books on DVD authoring, and spoke at the NAB 2005 Post Production conference so I have quite a bit of experience in this area.
- Mark