Buying 2011 13in Air - not sure

i'm considering buying loaded macbook air 1.8 i7 4gb ram , 256gb ssd.

own white unibody macbook 2.26 core 2 duo 2009, 8gb ram , ssd, , reasonable 5 7 hour battery life, find carrying around time killing back.

upgrade? 3 years later macbook air still offers 7 hours. how appreciable increase i7 core 2 duo 2.26 , air's faster internals? should wait ivy bridge , usb 3 , maybe faster processor , longer battery? 4gb ram on air enough computing in next couple of years? want buy new computer last me @ least 4 6 years.

don't need upgrade @ instant, want to.

 

what sort of applications use? use computer for? etc.

 

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