Quick Answer:
I recommend just getting the Asus GTX750Ti:
http://pcpartpicker.com/part/asus-video-card-gtx750tioc2gd5
Long Answer:
1) For video editing, the Graphics Card usually has very little impact. It's the CPU that is usually most important. It varies between programs, but I believe of the programs that even used video cards much the advantage going above a GT640 was minimal.
2) 4GB is sufficient for most video games, but not for video editing.
3) By video "editing" I don't mean simply converting video either. That's a different story which uses the CPU only for software-based conversion like Handbrake (the best quality conversions). Many programs can now use the GPU decoder in graphics cards which can do a fairly good job and are much faster, but I'd use it for converting to watch on a small screen.
4) DDR2 memory is fairly expensive.
5) You must have a 64-bit OS to use more than 4GB of memory. That includes the System RAM (DDR2) and the Video RAM.
*If you have a 2GB video card and 32-bit Windows, you will have less than 2GB usable of your System RAM.
Summary:
I'm not sure what to recommend really as you need more System Memory for video editing but also lack a great CPU. As said, the R9-270X would be bottlenecked severely, as would some lesser cards though how much varies between the games as well.
So I can only recommend:
a) GTX750Ti to improve gaming performance, or
b) Build a completely new system.