Hi Zanzaboo, I'm not aware of anything we've done to specifically disable unsupported video cards, though we definitely cannot guarantee their compatibility nor have we tested or validated them in any configuration and for that reason do not recommend them. For supported graphics cards on the T420 (which you have correctly identified as the Q4000 and Q6000), you must disable embedded video output from the BIOS/System Setup under Integrated Devices (pg 30-31, support.dell.com/.../T420OMen.pdf ) to get video out. Note that when you do this, the iDRAC virtual console video output will no longer be available until embedded video is enabled again. I don't know of any reason you cannot follow the same steps with an unsupported graphics card but I must stress we did not test or validate it and if it doesn't work you would not have any recourse under warranty (i.e. we would not replace the motherboard for a faulty PCI-e port, for example). I'm not sure if PCI-e video passthrough will work on here, but you may need to configure the SR-IOV setting (right below the embedded video setting) per the needs of your hypervisor. Let us know how it goes, and good luck!
I recently got a T420 for Desktop virtualization in a lab environment.  I'm not doing VDI but I am going to be running multiple VMs at a time, one of which will be my daily work VM.  I have no need for a several thousand dollar GPU but I would like better graphics than the on board [] that most servers are given.  No crazy gaming but some GPU capability would be nice.  Here is the rub.  It seems the ONLY cards this guy supports are the Q4000 and Q6000 Nvidia cards?  Seriously?  Is this really the case? I don't care about Dell supporting it but I am not allowed to use my $100 Graphics card from Best Buy?  I really hope I'm just missing something.  Can the T420 use graphics cards other than the Nvidia Q4000 or Q6000?  If not, does anyone know why?  Is this a bios enforced limitation?  Is it some other technical limitation?
T420 Graphics support?