Newbie storage questions... (RAID5, SANs, SCSI)

Hi,

    I'm reading a book that describes how to plan an SQL Server
installation. The book warns that one should never use RAID5 unless the
volume receives less than 10% writes (i.e. >90% reads). Apparently the
performance penalty for data writes is quite high with RAID5 but I'm having
trouble understanding exactly what the penalty is. Consider the following
example:

    - Let's say it takes x seconds to write a chunk of data to a single hard
drive in a single IO operation.

    - Now let's say that I have 3 of these drives in a RAID5 array and I
want to write the same chunk of data. Instead of using a single IO
operation, four operations are now involved because to write a bit of data
to a drive, the RAID controller must:
        1) read the preexisting bit of data on the drive
        2) read the preexisting bit of data on the parity drive
        ...calculate the new parity bit and then...
        3) write the new bit of data to the drive
        4) write the new parity bit to the parity drive.
    ...although there are now 4 operations, the operations are spread over 3
drives. So the time to perform this operation is [4x/3] seconds, that is to
say, 1.33x seconds or 33% longer than it would take to write to a single
drive. Using this logic, if I had 4 drives the write speed would be
identical to writing to a single drive. Only when I have more than four
drives is the write time of the RAID5 volume superior to writing to a single
drive. Is this logic correct? This is how my book describes it, but in
practice RAID5 doesn't seem to be as slow as this. Could someone please
confirm?

    Now let me ask you a question about bandwidth for data transfer from
hard drives. Consider the following device that Dell sells which appears to
be a stand-alone rack-mountable RAID device:


http://www1.us.dell.com/content/products/compare.aspx/sanet_fibre?c=us&cs=555&l=en&s=biz

    This thing costs nearly $12,000, but yet its bandwidth of 200MB/sec is
inferior to standard SCSI which is 320MB/sec. My question is: What is the
advantage of this device over traditional SCSI RAID?

    One last question: The SQL Server seems to be a single point of failure.
If the motherboard or power supply in the SQL Server machine goes down, my
entire application will go down. Is it possible to set things up in such a
way that TWO machines running SQL Server can be attached to the same
harddisk? If one SQL machine dies, the other machine automatically comes
online? In this scenario, where is the harddrive located? It can't be in one
of the machines because if that machine were the one that crashed, the other
SQL Server machine would be unable to access that drive! That's why I'm
wondering if that Dell external RAID solution (see above) might be
appropriate for me. What do you think?

Thanks,

David