It's a common amateur mistake to look at the spec sheet when choosing a graphics card. If you go by what gnomio said, the 6750 is about 25% more glfops but real world performance would be much less, closer to 15%. You want to look at real world app benchmarks not spec sheets. http://www.overclockersclub.com/reviews/sapphire_hd6670_ultimate_hd6750_hd6770/
You're mostly only going to find gaming benchmarks for graphics cards but if you have gpu acceleration when video encoding it will speed it up with a more powerful card.