For example, I know GT640 is better than GT640M.
1.Is GT630 better then GT640M?
2.Is GT620 better then GT640M?
3.Is GT610 better then GT640M?
How to check for their performance if I don't have any card before I want to buy?
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If you don't have the card,
Research through internet. Read reviews and compare the posted benchmarks.
A list is over here - http://www.notebookcheck.net/Comparison-of-Laptop-Graphics-Cards.130.0.html