They usually support the most recent 3-4 generations of graphics cards at once.
There is no newer generation card than the 5870 out yet, so support isn't an issue for it.
Pretty much the decision to wait or not should be if you think the HD6870 will be worth waiting for instead, since it comes out October.
They aren't pulling the plug on the graphics division, they are only putting it all under one name. From now on you will buy AMD video cards. It's that simple.