Length of SATA cable for a full tower

Looks like the SATA cables in the motherboard box should be plenty. how many drives you plan on using?

To paraphrase Lincoln, the should be long enough to reach the drive.

SATA spec is the cables should be between 30-100 cm. Shorter than 30 cm (12 inches) and you may have interference problems with internal reflections. Longer 1 meter (40 inches) and you may have problems with signal degradation. PCB traces from the motherboard's SATA controller to the socket, and on the drive have to be included in this length. So 30-36 inches is a safer maximum.

Most people like to use cables which are long enough to reach from the SATA ports to the drives, with the table tucked onto and cable tied to the edges of the frame, but no longer. That way they're both out of the way and do not impair airflow, and you don't have excess cable taking up space.

Having just swapped my case out I can attest to that it depends how your case is designed.
Original case https://www.amazon.com/gp/product/B008ANFQ8U/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

New case https://www.amazon.com/gp/product/B01JMVLBUA/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1


My old case had hang down cabling, because the mobo hung along one side, and while I could tie down best I could still left (as said above) pretty ugly airflow. The new case puts the board horizontal and then feeds the wires below it, which is incredibly smart IMHO. So now I can wire no issue, max airflow across the BOARD (it isn't just your CPU and GPU you need to cool) and still have easy access to stuff

Okay tossing "bigger numbers" at things in computers doesn't mean you get the result you want. What your asking to do is a big problem and NOT FEASIBLE.
It isn't "I need more RAM" but there is alot of parts here we are talking about that do alot of different things for different usages so let's break down a bit here.

1) You don't need 8 Drives, in fact there is really no need for it unless your running RAID arrays (which is a different discussion) to have multiple drive failover and 'hot swappable' drives.

2) COD MW3 no one has played in years and doesn't take any real 'hardware' to play with the current gen systems. BO2, does take some hardware and GTA V of course needs a solid base - BUT it all depends HOW you intend to play these games.

Are these / other titles going to be played on a single screen?
Are you expecting a particular resolution? Graphics detail level?

3) If you need a "video encoding and conversion" AND you want to play video games, then you need TWO systems, not one. Video editing is CPU intensive, and yes the MORE memory you have to that task the better, but really you also need high speed drives, most take advantage of a SSD as the 'work / OS' drive then have a second or third (depending on how much production they plan to do before archiving the work to the NAS) to 'move' the work to when done to store for the time period to use that video then archive all the videos.

4) For the titles you mentioned, the "gaming system" you can even just stick to a Console if you wished, and yes you can capture to the Digital Editing PC from there the cable box, whatever with a simple Capture Card. Otherwise a simple "Walmart" i5 and toss in a 1050 Ti (so no mussing with the power supply) and you can game all the current titles or ancient ones like you mentioned for a single screen 1080P usually Med / high Graphic detail for 40-50FPS performance. But (points up) depends alot what you 'expecting to play'.

5) "I am not computer smart." - OKAY?!? Your tossing out stuff like you 'know' something, that is a start but what your 'tossing' is basically yourself with a 100lbs weights tied to you into the DEEP computer smart ocean area. This is sort of stuff your asking to do, is not a "appliance" you can just buy and "figure out" over time, alot of this is learning the basics first so you don't blow wasted money (hundreds of dollars) in mistakes and errors. If you really don't know ANY of what I just replied back with above, then STOP for a second. Go to the local bookstore and get the Idiot's Guide (they are awesome series) to CompTIA A+ Certification. The A+ certification covers EVERTHING A-Z on computers, and once you know that 'basics' then you really can start making better judgment on what you need and how to achieve it, because YOUR ON YOUR OWN WHEN WORKING ON PCs. YOU ARE EXPECTED TO SOLO FIX AND DEAL WITH ALL YOUR OWN ISSUES (WARNING Will Robinson! Warning! - LOL). There is no simple step by step, or ABC click here do this done solutions ALOT of it is methodical, logical step by step process of elimination.

I hope this clarifies things more.