DO NOT DO THAT! I repeat, DO NOT DO THAT. An ordinary HD 4890 has 2x 6-pin power connectors. XFX4890 Black edition has 1x 6-pin and 1x 8-pin connector. After restart it won't let you get even to BIOS! It will just flash a warning that the card isn't getting enough power.
* * *
Anyhow, try...