What is this?

My friend asked me to deal with her laptop because she was saying that her laptop kept on crashing. The model that she had was the Sony FZ21Z. It is virtually brand new and she asked me to help her. When I went through her laptop, I reinstalled all her drivers except for her graphic driver. I used a programme to see what sort of graphic card the laptop has and guess what? It showed an Nvidia Geforce 8400GS! I thought her laptop should be 8600GS? That is a question.

I have then used a 8400GS driver to install on the Sony VAIO FZ21Z and everything seems to be working much faster than before! What happened? Is it a 8600GS or a 8400GS?