Steve - other thread in this section about scratch disks. As you've used the Revos a bit - thoughts on using a smaller one (perhaps 80GB to keep price down) for scratch disk compared to RAIDing a few regular (or SSD) drives?
This really depends on the size of your work, RAM, machine configuration, desired speed, and available space on other fast storage devices.
For instance, if you're just processing 2-3 images at a time, say limited to 3-4 layers each, and you have 16gb of RAM and an extra 30gb on your system SSD.. most people would be very happy with the results.
If you're rendering a lot of video and need a larger scratch disk, then a dedicated SSD becomes practical.
I'm on record saying I'll be spoiling myself and using a Revo 1tb hybrid for my LR indexes and previews.. and I'll also put my scratch disk on the same drive. The hybrid is perfect for LR where you'll be accessing the same galleries/collections as you work, and a scratch disk which is active.
Others will be happy to move their scratch disks to an archive disk rarely used. You don't really need a separate disk, just a disk not being used in the course of your normal work flow. Of if it is used, if it's very fast such as a fast SSD it will be fine.
I didn't respond to that thread because it seemed like they were looking for a one size fits all answer.. and while I can give you a generic answer if money and PCIe slots are unlimited, for most of us they are, and the answer depends on your machine configuration, your work flow, RAM, and other storage devices..
It's one of those areas where you have to think your way through it based on your individual factors.
I'll bet John has some good thoughts on this too.