for NVIDIA cards (the ones you chose)

it goes like this

the first number is the generation

the second number is the series

the third number is the sub-series

and the letter means it is a mobile chip

each generation is about 10% more powerful than the same card a genration back

the 260 is about 10% more powerful than the 160

each series is about 5% more powerful than a lower series

so the 260 is about 5% more powerful than a 250

this means the 840m is easily 150% more powerful than the 260...

Passmark Comparison

http://videocardbenchmark.net/compare.php?cmp[]=83...

You can actually compare benchmarks of any of the cards using a website like http://www.videocardbenchmark.net/

(Obviously nvidia mobile cards will have an M)

Typically the best benchmark scores indicate best gaming performance.

This link compares the two cards you asked about

http://www.videocardbenchmark.net/compare.php?cmp[...

The 840M is much faster!