Sometimes you just need more RAM to stop it getting bogged down.
when i do "presentation" rendering in autocad it can easily eat more than 8gb so ive gone with 16gb in my rigs

Sometimes you just need more RAM to stop it getting bogged down.
What use could you possibly have, PC's can only get so fast, it's still going to take 30 seconds to boot and ages to do a file transfer on a standard Hard Drive. How much performance boost could you get increasing ram from 8 to 16, I highly doubt it's twice as fast
I guess for stuff like graphic design when you're dealing with RAW files that would be ram intensive, but for a home computer just for gaming and video encoding, I definitely couldn't justify more than 8GB, with iTunes, MS Office, Photoshop and Firefox open, my PC only uses 2GB, I'm trying to figure out a way to make use of the extra ram, to make my PC was quick as possible
8GB is plenty. Why would you ever need more?
You'll be fine using all 4 slots assuming the motherboard supports 4GB per slot.
It shouldn't really be any less stable and it would help with video editing.
To be fair unless he's doing some real heavy stuff 8GB RAM (2x4) should be more than enough. You could always stick the other pair in at a later date.
I edit HD footage and use 3d applications and have never ran out of memory so far and I'm using 6GB.