Do you know whether you have integrated graphics or a separate card?
Either way it sounds as if the display system uses some of your RAM for itself.
Did you put in the new memory chip before or after the failure?
If the stuff on your HD is as valuable as you say it would be wise to remove the HD temporarily and either put it in another computer as a slave, or in a powered USB case and backup its data contents before anything worse happens.
Above all, don't try and interfere with the OS and contents of the HD until you have a normal display again.