I'm making an ass
umption here that the VDDC current listed in Gpu-Z is only for the GPU core and it's circuitry.
You also need to include the MVDDC current and it's usage to factor in the total power consumption of the video card.
The HD4870 has been tested by Xbitlabs as using 65.2W during 2D IDLE as seen below:
My HD4870 running at 500/900 @ 1.263v (2D IDLE) reports a VDDC current of 25.7A, in Gpu-Z.
25.7A x 1.263v = 26.963W
65.2W - 26.963W = 38.237W
You can figure that the HD4870's MEMORY is consuming most of that remaining 38W.
This would be a great explanation (if true) as to why underclocking the MEMORY on the HD4870 saves so much regarding power consumption, while underclocking the GPU does very little...