XPS 8900, 16GB ram upgrade?

I have a 4.5 year old XPS 8900 (warranty expired on January 23, 2017). Four RAM slots. Came with 2 x 8GB (XJX08 : Module,Dual In-Line Memory Module,16GB,2X8G,2133,N-E).  The speed is 2133.  I'd like to add 16 GB for 32Gb total.  Need RAM for keeping too many browser tabs open.

Can I do one 16GB module; presume I need 2 8GB modules?

Does speed need to match existing 2133 or can/should I go faster?  Thanks.