What video editing programs do you work with? I believe video editing is not that hard on your graphics card so you should be able to get a cheap 4670 or 5670 to run in your system. If you are doing any 3D rendering however you will need a much more powerful card maybe even a workstation card depending on the programs you run.
You actually don't really need a good video card for video and photo editing. A video card will reduce rendering times but that is more heavily reliant on your processor than video card. Any video card should do.
You might want a card with multiple outputs if you use multiple monitors when working.
Most important thing is what app you work with and what monitors you hook them up to.
If you use DP or HDMI 1.3 enabled monitors or multi-monitor configuration it matters a bit, if you use apps that prefer one or the other architecture it matters, but a majority of the apps out there do their acceleration via OpenGL and it's available to both in one way or another.
Main ting to look for is good quality mid-range with low power requirements. I wouldn't bother with an HD4850, too much power consumption, a better choice in that space would be the HD5750 or HD5670.
My personal preference would be for an HD5670 which gives you a balance of performance and power consumption as well as new features.