Borderlands 2 "Running out of video memory! Exiting."

BOUGHT borderlands 2 on steam. while i enjoy the game with my friends. the game crashed already 3 time with the massage

"Running out of video memory! Exiting." last time took 2 mins to get that...

i should deal the game very easily with my computer due to its a not so high setting game.

my computer is:

i5-3470 Ivy Bridge 3.20Ghz

4g RAM

GeForce GT 640 3.5g video memory

Windows 7 32 bit

but i saw poeple with stronger computer have the same problem. im not playing on the highest graphics setting but it still happens. what should i do??