1) Dell's Support web site.
http://support.dell.com/ After logging
in, enter your PC's serial number in the Service Tag field.
2) The invoice that came with the computer will specify what you
purchased. You'll want to compare this to what your original order
form, to make sure that you got what you paid for.
3) The box the computer came in usually has this information on a label
on its exterior.
4) Start > Run > MSinfo32.exe.
5) Start > All Programs > Accessories > System Tools > System Information
6) Start > Run > DXDiag.exe.
7) Right-click My Computer > Manage > Device Manager.
8) If none of the built-in tools provide the information you want, you
can use these utilities to help determine the answer to your question:
SiSoft's Sandra
http://www.sisoftware.co.uk/index.php?dir=&location=sware_dl&lang=en