If you are working with one layer, the drive speed won't matter too much at DV25 (5:1) which is what standard miniDV transcodes at. When you work with multiple layers, you need the faster drive so that it can read multiple files concurrently and keep up to speed so that everything is in sync and running at standard fps. If you can't keep the sustained rate fast enough, the video editor will slow down the fps to keep things in sync using a buffer for the frames. So it is still workable, it's just that it's a bit annoying to work out of real-time. You can always render layers together (the equivalent of an audio mixdown) if you need to, thereby making it a one-stream read-back. If you are doing simple stuff (maybe just piecing video around and maybe some titling) you'll be fine at 5400rpm.
The other thing is since miniDV at DV25 is already compressed on tape, you don't have to worry about frames being dropped during capturing because you aren't really capturing, you're transcoding. The difference is during capturing/digitizing, you're actually sampling and compressing (or not) the media as it is coming off of tape and placing it on the hard drive. If the hard drive isn't fast enough, then frames will be dropped from the capture since the computer is dealing with an onslaught of information coming off of the tape. Since DV is already in its final compressed format, it is only trasferring the media during the "capture" process.
thanks