First I'm sorry about your mother. Cards can be such a blessing when you receive them - I think its a good idea to save them for your sons. A memorial is a good idea.. Maybe you could find a special box that could include the cards and maybe some pictures of your mom like wedding pictures, and other family pictures or mementos so that they would have not just the cards.. But also bits that showed her life. maybe you could even write up some of your own memories about her.. Something like that might be nice to pass down to their own kids one day even.. So they know their great grandma a bit.
You could make a memory book....like a scrap book. You can include the funeral ceremony and newspaper write up, I would keep it at your house and when your boys visit they can look at it. If one boy seems more interested in it you can leave it in your will for him.
I read something about someone saving all of their sympathy cards in a decorated box with a picture of the person on the lid. And every 10th anniversary they would bring them out and read them.