Fire up Task Manager while you're playing, wait for your framerates to bottom out and check memory usage in Task Manager. If it's not maxed out, you're not going to get any benefit from more memory.
I'd wager that memory is not your problem, but the video card struggling with rendering tons of objects at once. Even nice cards like yours can get bogged down in situations like that.
I'd recommend overclocking your GPU to try to boost your framerate.