Always go for the 2nd number in either nVidia or ATI GCs. The greater the 2nd digit, the better the card is.
There are exceptions though, like ATI's 6000 series over the 5000 series.
In your case, 570 >> 640
The T&C's explicitly state no returning laptops under the satisfaction guarentee.
Also I checked out Canadas consumer rights laws and they are not there to help you at all. Compare them to the ones here in the UK and you will see that our draconian laws are actually much better at protecting consumers.