I wonder if the motherboard has a eeprom that only let you use Dell's proprietary bricks.
A GTX 560 will work, Dell/Alienware have an official video upgrading the X51 with a GTX 560
The only issue with that vid is that they neglected to show any tests or results. I want to know if that card will run completely stable in that machine. It probably will, but I appreciate all the info I can get my hands on.I just posted the video in a new thread here: http://www.alienowners.com/showthread.php/122-Alienware-X51-Video-Card-Upgrade-Video