Hi there, hope I can help. Go to forums.us.dell.com/supportforums/ in the upper left, click "technical support" > manuals > dimension desktops scroll down on the right side to XPS R___ Look at specifications about your memory, lot's of good reading. Somewhere it will tell you how to open your tower and how to replace memory, etc.
You can also enter your service tag (the little black tag on the tower has your number) and find out everything installed on your system when you bought it.
Also open "Community Forum" > dimension message board > upgrade hardware and scroll down to bottom of first page and type in XPS R in the "Search this board"
I used crucial for my new ram. You can go to http://www.crucial.com
and use the make and model selector, if in doubt, you can call them direct. I had to call because their search said I had 3 slots on this Dimension4100 and I knew I had only 2.