For single card storage, I've seen the plastic card protectors that go in a 3 ring binder. Some of these say archival quality, but who knows. Is it certain types of plastics that are good, or is it a coating that makes it archival?
I've also seen the hard acrylic single card holders. In general are those ok to use, or some archival, some not?
In general, fir protecting cards individually as opposed to in a tuck box or as a whole deck, UltraPro is the industry standard used by collectors of sports and custom gaming cards. Certain plastics have a chemical that can trigger premature decomposition of the paper, but UltraPro's products use a completely pH-neutral plastic. Their products are easily spotted - they're all marked with holograms of the company name/logo.
I've used the hard acrylic holders myself - but I've used them with UltraPro card sleeves between the card and the acrylic. Not only is it an easy fit but it makes later extraction of the card from the holder simpler, as the sleeve will usually extend just enough past the opening to allow one to pinch and pull the card out without even making contact with it.
In a previous lifetime, I was a collector of CCGs. I actually had a friend who got into "Magic: the Gathering" back when First Edition was still for sale and Arabian Nights (the game's first expansion) had only just been released.