If you have 3 sticks of 256, or a stick of 256 and a stick of 512, then you have 768.
How much physical memory you have is different from video card memory.
Be aware that you can have SERIOUS compatibility issues if you put a non Dell part in a Dell.
Sometimes you can get away with using non Dell parts, and sometimes you can't. I've gotten away from fooling with proprietary machines like Dells, Compaqs, and HPs.