Adding 1 stick of memory will make it so you do not have Dual Channel memory. Why not just get 2 sticks? 4gb of DDR2 is cheap right now.
You have a nice video card and I'll bet it games pretty dern well now. If a new game comes out that needs more horse power, I'd say upgrade then, who knows what the optimal video requirements will be for it. There might be a new video card out then that does better than your 2 4870s and you may end up spending as much for it, or a little more, as you would spend on another card now. But hey it's yur money, if you have the extra cash, spend it how ya want.
Make sure your power supply is up to the task of running 2 video cards first.