You should not oversimplify such considerations. The question really is, how you handle your footage and what your render settings are. This includes things like multiprocessing, naturally, but even for conventional renders it matters where and how the items are stored and what format they are in. Yes, a RAID may help, if you use lots of footage and that footage is stored on the drive, so reading the frames goes faster, but it may not necessarily help with storing the files, as only every few seconds or minutes a framer will be written. In reverse, a RAID could help, if you are outputting the same clip to different formats just by adding multiple output modules to the same render queue entry, but that one is rare. One other possible improvemnt would relate to the disk cahe, but it's really mostly only relevant for RAM previews. It also becomes a question, of whether AE actualyl fills it in the first place - if your project is not structured accordingly and AE thinks, it can calculate your effects faster than reading them from the cache, it will not use the disk cache for this stuff... It's really much more complicated, so you should evaluate your options based on the pointers I tried to give you.
If you are using many footage items in your Comp, a RAID will very likely improve rendering performance because reading from disk will be faster. It depends on the format. HDV and AVCHD have actually lower data rates than most SD formats, but are quite expensive to decode. If you have a ton of these, a RAID could help but the decoding stage will also also be a limiting factor. For other formats (DVCPRO HD, even footage from the famous Canon DSLR cameras), the advantages may be more clear.
For everything but the most simple Comps, my opinion is that write speeds will hardly be an issue. It's unlikely that AE will render frames at more than the typical sustained speed of a SATA disk. Most advantages will problably come from the increased read speeds.