If your IDE drives are fast, I don't see why you would need to switch to SCSI.
Just a warning---I don't capture/edit raw, uncompressed video files (AVI's)--I capture in real-time to an MPEG2. If you were working with raw AVI files, you may want to go to SCSI. However, for converting video tapes that is probably unnecessary since the quality of the source video is limited anyway.
The time it takes depends on what kind of editing/filtering you want to do. For me, I am just making pretty much straight transfers of old TV shows from tape to DVD in a less-than-DVD quality format. I'll just tell you the steps I do and the approx time it takes so you have some idea. If you use different software or capture in different formats it may take more/less time.
1. Capture--in real time. So a 1 hour show on tape takes 1 hour. With an Athlon XP 1700+, an AIW 7500, and a 7200 RPM Maxtor drive I don't have any frame loss.
2. Since I use ULEAD for DV editing, even though I capture the original file in an MPEG2 format it still wants to convert it. No big deal--1 hour of video only takes a few minutes since it is already in MPEG2 format.
3. Sometimes I'll crop the video into separate files or to remove some things leading/trailing as or credits, for instance) Cropping and then resaving the file again only takes me a few minutes. Warning--this time increases dramatically if you work with large files, mostly because of disk-swapping.
4. Creating the DVD files. Depending on what kind of filtering and the quality of video you want, this can take a bit longer. With ULEAD first I select the different video files I want included on the DVD. Then I create a scene selection menu by defining where each chapter should be (you do this visually--it's easy), then I let the conversion run and the DVD files get created. This is generally the slowest part of the process. 1 hour of video usually takes me about 2 hours to convert, but it may be more or less depending on your filtering, quality and of course hardware.
5. Writing to the DVD is then just the process of copying the files created by ULEAD and burning them to the DVD itself. Depending on the speed of your writer, this is also a bit time-consuming. Since this is the last step in the process, I usually just start it up and then go do something else. Depending on file size and writer speed, this can take 30-60 mins.
So basically, if I want to convert 1-2 hours of videotape to DVD, I set aside most of an afternoon or an evening. You don't actually have to sit through it the entire time since a lot of it is just sitting and waiting. Usually I'll break it up into separate sessions--start the video capture before sitting down to watch a ballgame on TV, then doing the editing/cropping, then start the DVD file creation process before heading out to the store, and then writing it to the DVD disc itself before going to bed or maybe even on another day.