System RAM does not do the same job as video RAM.  System RAM is used by the processor to temporarily store data that the application knows that it will be using later.  Video RAM is used as a high-throughput buffer for the GPU on the video card.  While system RAM does not take the place of video RAM in the case of an integrated video adaptor, on many low-end computers the system RAM is "shared" as video RAM.  In other words, the System may have 8 GB of RAM, and 2 of those 8 GB may be robbed from the system for the integrated GPU to use as video RAM.  But adding more RAM to the system does not give you any more video RAM.

     The computer that your link takes me to seems like a fairly decent system.  However, it doesn't match the specs that you described.  Keep in mind that the vodeo card quoted, while it's a nice card for gaming, it is just that:  a card for gaming.  It will probably provide you what you need for now, but it can't be looked at as a "multi year item."  As time goes on, and as you get more advanced, you'll find that this card lags and you'll be wanting something else, something that is taylored for processing video.  The Quadro line of nVidia cards are meant for graphic design and video creation.  The AMD equivelant would be the Fire Pro line of cards, however nVidia is the way I would go.  These cards cost a little more, but they are meant to do what video editors want them to.  As far as RAM goes, 16 GB is quoted in the link you provided.  That's actually plenty.  Despite popular belief, more RAM does not necessary equal more speed or more power.  It simply means that you can have more applications open at one time that require more system RAM.  However, if you're editing video, it's recommended that you don't do other things simultaneously that require lots of system resources.  So 16 GB of system RAM should give you more than enough to do what you want to.

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