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To answer your question yes, a dedicated video card instead of an on board graphics chip will definitely improve the SPEED of rendering your videos. Regarding the quality of rendered video no.
This does also all depend how much RAM have in the first place.
If you have a PC with 512mb of ram and onboard graphics of 64mb usage then you wont see much difference, if you have 1gb of RAM and 64mb of onboard graphics then youll get better performance, now with for example the top amount of visible RAM on 32bit machines is 3gb so lets say you have 3gb of RAM and 64mb graphics then youll have a good increase in job speed as the RAM that the onboard graphics is only making a slight difference to the 3gb you have.
So if you have 3gb of RAM and a Graphics card with 512mb of its own RAM then all this means is that you wont be using the 64mb that the onboard graphics would of used but you are utilising the whole of the 512mb of RAM on the graphics card as long as your processor can cache fast enough.
Basically for fast encoding/rendering you want a good amount of RAM and a good processor and if using onboard graphics just make sure that you have enough RAM.