yong, did you ever nail down a max current for the 4890? if not, i would just create a Y cable that pulls power directly from the power supply, and then hitch the PWM/tach from the other side to the GPU fan header. this keeps the RPM and PWM connections live, but doesn't tax the power circuit. I made an adapter that plugs directly into the GPU PWM header, and converts it to a standard motherboard PWM header. it wouldn't be too hard to splice off the power from that to another connection of your choice.