streaming processors are like cores almost. But that doesn't mean 480 cores. Because Amd bundle them together they are way lower than that number actually states.
To make it simple think of it as the horsepower of the gpu. But for that horsepower to work you need a good connection to the memory of the card as fast processors and slow Vram can slow down your gpu and vice versa.
The easiest ,way to see what gpu is the best just look at its SINGLE PRECISION G/FLOPS.
The bigger the number the better it is
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... 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.