upgrade von OS X zu OS Sierra 10.12.5
How to proceed on a Mac Pro 3.1 with 6GB to upgrade
Do need more GB ,8?
and how to get it done?
Mac Pro, OS X El Capitan (10.11.6), memory 6GB
See the requirements here, and also how to download and install Sierra: How to get macOS Sierra - Apple Support
The MacPro needs to be a model Mac Pro (Mid 2010 or newer) - I suspect your model is older.
And the minimum storage requirements are:
It also requires at least 2GB of memory and 8.8GB of storage space.
You will probably get a better performance, if you upgrade the RAM to at least 8GB, better more, but it is not required to be able to install the Sierra upgrade.
Just take the usual precautions and make a full backup of your Mac before upgrading, so you can revert to your current system if need be: macOS Sierra: Revert to a previous macOS version
And check, if all software or device drivers you really need can be upgraded to versions, that are compatible with Sierra.