It does come with at least two sata power cables for more than one hdd. No worries you should have what you need (sata power) already laid out in case for you.  sata data cable is standard.  If not included, you can use any sata III data cable.  See pic below there is factory default sata power to upper left n right hdd bay [IMAGE]  
I have an XPS 8950 on order that will be delivered soon.  It is configured with a 2TB SSD plus 2TB hard drive and a 750w power supply.  I plan to install a Crucial 2.5" MX500 SSD into the second internal drive bay. I have a 2.5" to 3.5" drive tray--a rubbery thing from eBay.  I gather the 8950 will not come with either a power cable or data cable for the second internal HD.  Can someone please tell me what cables I'll need--power cable and data cable? Thank you. John
XPS 8950, cables needed for second internal hard drive