Ok, so I built the box, I rebooted, I got the twinkly array of AV Wizard noises followed by black screen.
Hit 2 -twinkly beep beep, black screen. Hit 7, twinkly beep beep, black screen. 2,6 twinkly beep beep, black screen. Getting the picture? because I'm sure not going to.
Using VGA, monitor definitely supports both resolutions.
I am using an NVidia Ti4800SE with UI2. I know 4xxx is not supported but worked fine in 1.1 Also works fine if I swap to another TTY, fire up xwindows and run the linuxmce startup sequence (even runs and completes the damn AV Wizard).
Save the settings, reboot the system..... twinkly beep beep ----- black screen
As Zaerc suggested above I would follow the suggestions here in section 3.1 ; http://wiki.linuxmce.org/index.php/Display_Drivers#X-windows_won.27t_start_for_AVWizard
Also it might be worth posting the video related settings that you have in Web Admin ->Wizard -> Devices -> Media Directors for the MD at the Core/Hybrid ie the following;