Mine shares system memory too - it's Vista/7 feature. Don't think you need to worry about it - it's automatically managed, Windows will reallocate memory as needed.
Something I learned from my time with linux that MS is finally coming around to.. Free RAM is wasted RAM. In linux RAM is always nearly 100% consumed. If an app or the system doesn't need it, it'll start sharing it with video or tossing copies of recently accessed files in there. If an app tries to allocate it, instantly free it up. From what I've read this is a behavior that MS is trying to emulate in the newer OSes, to make the most of your resources. RAM is the fastest storage in the system, might as well use it when it's sitting around.