Even if a file you are working on goes to 750mb why would one need a 150gb 10,000rpm hard drive for a scratch disk? I'm not saying this is wrong. Just asking, as my knowledge of an optimal set up is not that great. Photoshop and or Lightroom go to the scratch disk only after using all the space on whatever drive they are hosted on or do they go immediately to whichever drive you designate as the first scratch disk?[{POST_SNAPBACK}][/a]
Craig - Photoshop will use whichever scratch disk you have nominated as the first to use.
Edit/Preferences/Performance and then select which disks you want PS to use and then you can change their preference order.
The Raptors are certainly good, but you will obtain the "best bang for buck" just putting in a dedicated SATA II drive and put nothing else on it. An 80GB drive is extremely cheap although you may want to (literally) spend a few dollars more for a 160 or higher as they tend to have a larger cache on the drive (more performance). The Raptors are faster (and a solid state drive even better due to access times, but there are still a few issues there not least of which being cost) but for virtual memory work (which is what the scratch disk is) the benefit will be marginal compared to the additional cost.
If cost is not a factor, you could use 2 x Raptors in [a href=\"http://en.wikipedia.org/wiki/RAID_0#RAID_0]RAID 0[/url] but realistically it's not going to affect your workflow unless you're batch processing massive amounts of data.
You do want this to be a different drive to that being used by Windows for its virtual memory (and that, in turn, should be a different drive to your operating system drive and data drive) for maximum performance.