Is 24GB of DDR3 RAM overkill?

I have one of the old-school Core i7-920s and currently have 6GB (3 x 2GB) of DDR3 RAM running at 1600 MHz. I was wondering since prices are so cheap now, if I should just get 6GB more for a total of 12GB or just get 24GB (6 x 4GB). Or will that be overkill?

I often use about 70% of my RAM with my browser, mail, and all my background apps running (online backups, media streaming, etc).

So with 12GB, it would already drop to about 30%. With 24GB, it will be like 17%.

Anyone here use more than 12GB in their system? Do you see any benefits to all that ram?