I have an XPS-8910 on the way from egghead.com.  The PC has the default 16gb of RAM already installed.  Is one slot filled with a single, factory 2133mhz stick or is it (2) 8gb sticks?  Can someone provide me with the specs on the supplied memory stick?

Will there be any negative side effects be experienced if the other (2) additional 16 gb sticks I purchase are from a different vendor? ie GSkill or Kingston. 

Will the new memory take a performance hit by the existence of the pre-installed 16 GB of Ram already installed?