gtx 560 or gtx 650 or CPU

I have this computer:

O.S. Linux mint 18 cinnamon 64 bit.
linux kernel 4.4.0-21 generic
CPU Intel Core i7-4770 3.40 GHz
RAM 8 GiB
Gtx 650

and I have another computer with this:

O.S. Linux mint 18 cinnamon 64 bit.
linux kernel 4.4.0-21 generic
CPU Intel Core 2 Quad Q6600 2.4 GHz
RAM 4 GiB
Gtx 560

When I used cycles in the first computer, the CPU is faster than Gtx 650.
And When I put the Gtx 560 in the first computer and I used cycles again, the Gtx 560 is faster than CPU and Gtx 650.
I thought the gtx 650 is better than gtx 560.

Can someone explain me what it is wrong?
Should I buy a new graphics card for my first computer?
If you’re answer is YES.
Is this graphic card right for my first computer?
http://www.memorykings.com.pe/producto/195130/video-nvidia-gtx750ti-2gb-msi-n750ti-tf-2gd5-oc