I am from Dell Social Media and Community. I will be happy to answer your quire.
XPS L702X can support from 3 GB to 16 GB.
The Dell XPS L702X has 2 DIMM sockets and 4 DIMM sockets.
It depends on what type of motherboard and video card, you have.
If you could reply with motherboard and video card you have, I will be happy to assist you further.
You may also click the following link to check your options: http://accessories.us.dell.com/sna/category.aspx?c=us&l=en&s=dhs&cs=19&category_id=4325&mfgpid=208393&chassisid=-1