Cache-man said:
the two raptors in RAID0 provide super fast access to all of your files and programs, while the larger drive can be used for storage, although I would reccommend to use it to back up your RAID0 array too.
I currently have a 74GB raptor and two 36GB raptors. I intend to get myself another 74GB and set up a raid 0 (striped) to create the fastest 140GB+ I can (with a system partition of say 20GB and the rest for games and apps).
The two 36GB raptors I'm planning to mirror (RAID '1') as a 36GB temporary backup drive for downloads and files that I need to burn to DVD etc. I reckon this will give me a very fast and secure setup. (It also helps when you keep limited HDD space because you learn to back stuff up quickly rather than filling massive drives).
I also have two external (80Gb and 40GB drives just in case).
Important things to note:
1. Raptors are expensive and not worth buying purely for RAID - I've got mine gradually over time as i could afford to..the benefits of striped RAID are not worth the outlay of two Raptors at the same time.
2. The more drives, the more power you need and the more cooling in your case
3. The more drives and cooling, the noisier things can get
4. The more drives and cooling, the more wires you end up with
5. If you want multiple drives go for SATA - smaller cable profile
That's my two-penneth on HDDs for what it's worth