if you need more info on the mac pro, just type in the serial number of the machine at this link
http://www.appleserialnumberinfo.com/Desktop/index.php
it will give you the detail spec of the type of memory in the machine and the max and spare capacity for it,
another good link is the following as it will guide you thru the best selection for the up grades.
http://macs.about.com/od/macupgrades/tp/2009-2012-Mac-Pro-Upgrade-Guide.htm
as far as where to purchase I've used http://www.macsales.com/
in the past and they delivered on time and had great pricing.
hope this helps.
cheers,
MarcB
