I have a laptop running Elementry OS Luna Beta 1 with a broken Nvidia graphics card. I was wondering if there is an option in Ubuntu to disable the card. I mean, is it possible to render everything with the CPU?
It depends on your laptop.
If the Nvidia graphics card is the only part generating the display: No.
You can still boot and log in using SSH, X, VNC and similar tools.
If there are multiple graphics output generating chips (e.g. when the laptop uses optimus technology which uses a dedicated GPU chip and the slower GPU part build into some modern CPU's): Yes.
You might have to disable the nVidia chips though. Sometimes you can do this in the BIOS. Sometimes you will have to work with the drivers. (E.g. only load the driver for the other (on CPU) part. Not the nVidia driver, not the optimus driver).
For completeness sake:
In that case we are back to the first answer.
Depending on the age of your laptop, you should have a integrated or GPU on your CPU die, which should, but isn't always, be accessible from the BIOS. In a desktop you can choose which GPU you boot off, usually saying something like "PCIe or built-in" that you can tick to change it and everything.