(fresh installation) should I remove Nouveuau before install NVIDIA driver?

(Ubuntu 16.04) (GF9600GT)

Should I remove Nouveuau before install NVIDIA driver?

You don't have to. Ubuntu will automatically switch to using the proprietary driver you install.

I also recommend that you avoid uninstalling nouveau, since there are sometimes problems with the NVIDIA driver installation. You might need to revert to nouveau while fixing the proprietary drivers. If it's uninstalled, it's an extra step (in a non-GUI environment most likely) to install them again.

Keep nouveau and install the proprietary drivers. You may want to check my Q&A on NVIDIA driver installations in 16.04 for a guide. (Yes, it says "Graphics Issues," but following the answer will help avoid any issues in the first place.)

Install drivers for this adapter this way:

sudo apt-get install nvidia-340