Your first configuration would be best.
"I was under the impression from what I've read that the scratch disk (which I understand is a drive the computer will use when it runs out of RAM to process filters etc like in Photoshop)"<<<</div>
In FCP, the scratch drive or 'capture scratch' drive should be any drive other than your system drive (with OS, apps, etc.) It has no relationship to RAM, while other apps may.
I've not worked with Vista or Bootcamp, so can't really advise on drive speed/performance. I do remember getting our first G5 with a 10K rpm/60Gb system drive and regretted not going for the 7.2K rpm/250Gb system drive for more internal storage, when I found that FCP ran fine on the 7.2rpm drives.
I keep a variety of 'media' types on my scratch disk, but try to limit it to my captured video files for organizational purposes. This makes it quicker to locate and trash the video clips when I'm done with them....NOTE: I will only trash video that has been captured with valid original source timecode....all other 'media' gets archived for future use, if needed.