RE: Quick Survey

RAID5.  The slower writes issue seems to be less of a concern nowadays
as the main reason for it - parity calculations - is rendered almost
irrelevant by faster RAID controller CPUs and decent caches. 

Also consider RAID5 vs. 1: Fewer disks not only cost less initially but
take less space, consume less power, and generate less heat.

On a modern system, the only major down side is data loss if a second
drive fails before the first failed drive is repaired/replaced.
Exceedingly rare.

If doing PC-class RAID on-the-cheap, you could run in to issues if the
RAID card itself fails.  Namely, if the replacement card will recognize
your existing parity set.

>From an app perspective, web serving doesn't do much disk I/O beyond
startup and reading pages into cache.  Back-end CGIs might do disk I/O,
but basic web serving isn't horribly disk dependent.  RAM for caching is
usually more important.