Type of motherboard & power supply on?

Here are the specs for your system:
http://www.dell.com/us/p/studio-xps-7100/pd
Here is the lowest priced RAM that will replace your existing sticks
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
G.SKILL Ripjaws Series 16GB (4 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9Q-16GBRL $180
The link is for 4 x 4Gb matched sticks. I strongly recommend you pay the premium and get the 4 matched sticks OR,
pull the 2 sticks you now have, read the label carefully as to speed, voltage, and CL (or look up the model # on the sticks and see if you can get 2 more just like them) and get 2 more that are identical.
Also, if you want to get two 2 x 2Gb kits, like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit Desktop Memory Model F3-10600CL9D-4GBNT $40
You can, just be advised that the risk of incompatibility rises slightly.
Your choice.