How to find out which NVIDIA GPU I have

If you have it installed, you can also use nvidia-smi which provides even more information for NVIDIA GPUs (this is, after all, what they use in the page you linked):

$ nvidia-smi

Mon Sep 15 16:02:31 2014       
+------------------------------------------------------+                       
| NVIDIA-SMI 340.32     Driver Version: 340.32         |                       
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GT 650M     Off  | 0000:01:00.0     N/A |                  N/A |
| N/A   59C    P0    N/A /  N/A |      8MiB /  2047MiB |     N/A      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Compute processes:                                               GPU Memory |
|  GPU       PID  Process name                                     Usage      |
|=============================================================================|
|    0            Not Supported                                               |
+-----------------------------------------------------------------------------+

If you just want the GPU name:

$ nvidia-smi --query-gpu=name --format=csv,noheader
GeForce GT 650M

please update your PCI ID database with:

sudo update-pciids

And use the following command in your terminal:

lspci -nn | grep '\[03'

You will see the model name of your graphic card. If it's ambiguous, you could search the PCI ID (something like [10de:11bc]) on the Internet for the corrent model name.