I wouldn't get an ATi card for video editing. If anything, OpenGL is used as a rendering engine so that's about it. But if a program is using CUDA, which is more common than ATi's API (Stream), then you're screwed. I got my 5770 for video editing a long time ago since OpenGL is what we use. But after a while, I needed to use other programs that used CUDA and I was pretty screwed.
But as of now, I have yet to see anything that actually utilizes Stream. There is NOTHING out there that uses it that I've come across for usage in day-to-day editing.