You may want to consider 10,000 rpm HDDs, making sure each HDD has a large cache, and supported Native or Tagged Command Queuing. (NCQ/TCQ).
You'll likely want to offer 2 - 4 GB RAM if it will have a largish database hosted on it. (OS Disk Cache uses system RAM, much faster than hammering the HDDs all the time.
If you are seriously considering suggesting RAID-0 tell the customer to just go elsewhere, or contact
www.Tyan.com directly.
Aim for the Intel Xeon 7030 or higher model. (2.80 GHz, large cache, dual-core per socket), with the money saved fix the base configs and provide us (on the forums) with some updated links to machines they can actually use ! (The above linked machines need considerable work on the spec, even on the paper spec).
The configuration links above are not good, they both only have 1 CPU which may cause PCI-X tunnels, etc not to be available (see mainboard documentation, to technical for a forum post to explain).
You haven't checked the onboard controllers support RAID-5/6, and if they do is it in software using a hidden service (ala: nForce Pro), or do they also provide a SAS/SATA or SCSI-320 controller.... if so how will it be used ?, you have no way to get drivers onto the system during OS installation.
How will they perform maintenence on the server w/o a CD-ROM / DVD-ROM drive to install the Operating System and run diagnostics from ?
How will they install RAID drivers with no Floppy Disk Drive ?
Do the HDDs being offered have 1 million hours MTBF (typical duty cycle), or are they just typical desktop HDD units that are likely to fail when under heavy load (long queues of requests, heaps of head seek operations, etc)., even with RAID-5/6 you don't want to go replacing the HDDs every 2nd month (for example).
Go back to the drawing board, read the mainboard manuals instead of getting someone else to do it for you.... although on that note if money is involved I would be happy to assist look for potential problems and 'solve' them... Ahead of time of course, so the customer need not be presented with, notices or encounter problems.
Update: http://www.intel.com/performance/server/xeon/platform.h... ; the website you appear to be ordering from to not clearly label, or even use the correct names, for the processors.... so you would need to confirm what you are actually getting.