The faster the drives the better, of course, but there also the bottlenecks to consider. I keep my programs, inputs and outputs on seperate drives; that has helped for rendering tremendously. However, for glitch free editing of multiple lines of HD (3 camera work etc.), your computer RAM and your graphics card(s) are perhaps more important (if you are already using an i7 or quad core CPU). For your new computer perhaps look at the I/O speed of the drive devices you are considering and check which graphics cards are compatible with your editing software. I use Premiere Pro 5.5 and there is quite of list of cards which that take advantage of the GPU for glitch free editing. I also have 24Gb RAM which has helped alot for RAM previews. I didn't start with 3 drives, two video cards and all that RAM. When I bought a new computer a few years back I made sure the chasis had room for extra everything AND made sure the power supply could drive all those devices. Then I added as I made money on video.
There are some work processes that have helped me reduce editing glitches. I don't add color effects. (i.e. adjusting colors so all the cameras look a little closer in color) until the end.
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