There are a variety of reasons to have more than one graphics card in a machine. The most obvious reason is the ability to have more than two monitors hooked up at the same time. The second benefit involves graphics horsepower. Simply put, two is better than one (mostly). You gain performance when using 3D rendering programs, and of course, with games.
Two 256MB cards aren't the equivalent of a single 512MB card as there is more to graphics performance than video RAM, most notably, the graphics processing unit (GPU) itself, which is sort of like a CPU, but has a specialized purpose. So you can see that having two cards means you would have two GPUs, and if you compared two graphics cards with the same type of GPU but with different amounts of RAM, then obviously the two card setup would beat the single card setup.
