It depends what sort of work you're doing, but I've found that nearly all of my renders are CPU-bound, not disk-bound. That is, the computer can read and write the frames far faster than it can calculate them.
Do you have lots of RAM? If you don't have at least 2GB per core in a multi-processing setup, your render could crawl while the computer swaps memory to disk and back. This is probably the largest factor for most folks in reducing render times.
Adding a dedicated render drive won't hurt, though, and it's certainly cheap enough.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production
