Oh... Audio work can very seriously tax your hard drives. A friend of mine is having really aggravating problems with his setup, and he's on a 3 GHz 8-core Mac Pro with 20 GB of RAM and a 1 TB hard drive
Granted, he also regularly uses that full 20 GB of RAM, and wishes he had more... but I digress..
The more audio samples you're mixing at once, the faster your HDD needs to be... (naturally CPU power is important too, as is memory) and honestly, although I can't tell you if this is true for you, seek times/random read/writes are often very important too, meaning you WILL want an SSD.
I don't know how heavy your workloads are, so like I said, I can't say for sure.. but it's likely that an SATA300 card and a reasonably fast SSD would save you a lot of headache.
RAID can be a pain, and as mentioned, a real headache, and unless you're going to do hardware RAID, don't even bother. In many cases RAID 0 with mechanical HDDs doesn't make as much sense these days, as you can get better speeds (and far better random speeds) with a single SSD on a fast SATA controller.
I suppose if memory isn't an issue, using a RAM disk isn't a bad idea. That's not an option for my friend.. as he's not only out of RAM, but working with 500 GB+ libraries.