Your December 2013 X51 R2 shipped with the following =C13, 125V, 6ft power cable330W Delta AC adapter brick As far as possible video card upgrade options, an Alienware Desktops Forum search shows the following threads that you can read through =X51 R2, video card upgrade
Greetings!  I recently upgraded my R2 with 16GB Ram, and am curious as to my Video Card Options.  I am currently running the following:-Geforce GTX 760 2GB PCI Express x16 Gen 2  -Intel Core i7-4770 CPU @ 3.4 GHz -16.0GB Dual-Channel DDR3 @ 799MHz (11-11-11-28) -931GB ATA WDC WD10EZEX-08W SCSI Disk Device (SATA ) -Windows 10 Home 64bit   I am unsure of the Power Supply, from what I've read I believe it should be a 330W for the 760 video card.  Thank you for your input!
X51 R2, upgrade options, #2