When asked where hardware dollars are best spent, I always recommend a great disk subsystem.
Hard drives are often the source of performance bottlenecks. Any dime spent on a high-performing hard drives will not go to waste.
Disks are also the component that will most likely fail. Matt, the Microsoft blogger, claims that disks have a failure rate of 2% per year per disk. If that's the case, then an investment in RAID arrays would be very wise.
As far as specific drive components go, it's difficult to make a recommendation without knowing the purpose of the SQL Server.
Here's Matt's post: