Setting up an old Dell 2650 and I'm getting video mode not supported immediately after Dell ROM stuff but before linux boot. I never even see the opening NST menu. I'm clueless to were this stuff is setup in Grub these days please advise.
Have you tried connecting a serial cable between your Dell and another system to see if you can boot the NST in serial mode?
Other than that, all I can think of is to play around with BIOS settings on your Dell and google on Fedora 20 and Dell 2650 to see if anyone has found any work arounds.
Sorry that I don't have a definitive solution.