I suggest getting something on the lines of the HD4850 / 9800gtx+. They are fairly low power consuming cards which should be plenty for video editing for a good few years.
Do you do anything else such as transcoding/gaming? The 9800gtx+ is able to GPU transcode, which is much faster than letting the cpu do it. For example, my GTX295 beats the Core i7 @ 4.35Ghz at transcoding a movie.
What is your budget? Is that 300W just for graphics or for the whole system?