Another good tip is to go to WINDOW > SETTINGS / PREFERENCES > PREFERENCES > UNDO and set the queue size to FINITE and somewhere around 100. Because what happens is, Maya keeps storing information on every step you take throughout the project and keeps pileing it up into memory. This backup of information can eventually take up valuable resources. By setting it to FINITE and a number somewhere around 100, you put a cap on the amount of information that is kept and it will only backup up to that number before it starts replacing older information with newer, more recent steps.

Also, you might want to get more RAM. 256 is too low.