Make sure you have driver version 190.38 installed,because apparently in old cards, the newest drivers are more memory aggressive, reporting less free memory to the system. You'll have to revert back to your older drivers if you're going to use a video card with the bare minimum amount of RAM.

But before that,just download GPU-Z and make sure that the CUDA checkbox at the bottom is ticked. Yes,people have large configs for cards, but that is needed only when you want specific configs,but the autotune is generally a really good option.