Try go to this page: http://support.asus.com/default.aspx?SLanguage=en-us
Hover your cursor over the text Support in the upper left corner then click 'CPU Support'
On the new site in the search field in the upper left corner write P5GDC Deluxe (don't write Asus) and click the 'Search Info' button.
Next you get a page with the name of your motherboard. Click that name and you get a list of all supported CPUs.