It will work with your board.
The Gigabyte 5850s have the power cables facing the side panel. For others, HIS included, the power attaches to the rear.
Your case will make all the difference whether it fits.
Your power supply should be fine considering only wattage, but what kind is it? Whether or not it has the 18A additional (just for the video card) on the 12V rail will make the difference there.