I need some information so that I can build a model cost model in
preparation for setting up a datacenter. I need sources that will help
me answer questions such as:

1. What are the physical space requirements per x000 servers? What
about per gigabyte of hard disk or of memory?
2. What are the power/cooling requirements per x000 servers? What
about per gigabyte of hard disk or of memory?
3. What are the main cost components, eg real estate, servers,
cooling? How do these break out in % terms, approximately?
4. For every $1 I spend on servers, how many $ will I spend on
everything else that is associated with a datacenter (see Q3).
5. Are there any key differences I should bear in mind if our
datacenter is going to be used for storage vs for processing power?

Thank you.

I need ballpark numbers, and sources that will help me model our numbers.

Your question leads to a LOT of assumptions.

a very good link to address the problem:

http://www.hpl.hp.com/techreports/2005/HPL-2005-107R1.pdf
(Cost Model for Planning, Developement, and Operation of a Data Center)
36 pages

Other things to consider Webuserid-ga, 

 
 - Humidification/Dehumidification Device
 - HVAC (Air Conditioning)
 - UPS
 - Backup Generators
 - FM-200 or similar DC fire counter measure
 - Powerfull Backup fans incase the HVAC fails
 - 2 seperate power lines for redundency
 - Rack Fan Setup (in/in, in/Out)  <<<< Referring to direction of air
 - Temperature sensors that are wired to a Fault Management System
 
Now all that stuff about the memory and HD space  is really (VENDOR
SPECIFIC).  You can have a double size cabinet with a storage capacity
of SCSI 16TB or the same cabinet with CPU blades and Storage.  Your
question is really vague but for $200 I guess you can.  If you Post a
more detailed clarification (e.g. vendors and specific requirements) I
will attempt to answer your question.

Good Luck, 

Who3ver-ga