The easy answer to most of your questions is NO.
- what does a graphic card do?
A Graphics Card is the device inside your PC where you plug your Monitor in. Your computer monitor not a video monitor. Some Motherboards have an onboard graphics chip, and you can plug your monitor into the motherboard to get a picture, but otherwise its impossible to run a PC without a Graphics card.
- does a graphic card improve the quality of final exported video or does the video captured by graphic card have better quality?
Firstly you cannot capture video with a Graphics Card.
It will not improve the quality (I might be wrong here, but I dont think so), but it can increase the speed at which your computer renders out the video.
- will there be difference in the final dvd exported from a computer with graphic card and one without graphic card?
Well, you cannot run a machine without a Graphics Card. And secondly no, your graphics card has no influence on the quality of the encoded video on the DVD disc. This comes down to your source material, format you edited in, export settings when you encoded the video to MPEG2.
Hope that enlightens a bit. Please ask more question if you are not clear.
thank you platduck very much for your enlightening information about video card.
it didn't enlighten a bit....it enlightened a mega-byte....thanks once again.