Which format do you using to edit the video (DV, HD, AVCHD)
OK, I answered my own question here, and for anybody interested, the answer is a resounding "no", it does not work very well to put your scratch disks on an EIDE drive (at least for DV-AVI source, which is what I was using)!
The time required to render previews was significantly more (subjectively, I'd say about double)
But, oddly enough, it was the *playing* of those rendered files that was the deal-breaker: suffice it to say that it was not just bad, or slow, but completely unusable.
I went to my Preferences, and moved all my scratch files to one of my SATA-II drives, and everything started working perfectly again.
You beat me to the punch. I was going to mention that the 5K RPM would not yield good playback. I doubt that the controller type had THAT much of an effect (I use a 1TB ATA-100 for some Scratch Disk work, but it's 7200 RPM), and have no noticable issues over my SATA II's. Now, EIDE is slower, but not by all that much. I believe that you could do as you wish, if that older HDD was 7200 RPM, but that is just a guess on my part.
Thanks for reporting,