I am thinking of running a variant Harn d20 game, replacing the standard DnD magic with the Call of Cthulhu magic and sanity system.
That sounds like what a lot of us have been suggesting for a d20 Conan game.
There is NO magical healing.
Please realize that D&D's rules expect magical healing. Without it, ablative Hit Points dwindle rapidly.
Almost all magic will appear to have no visible effect or at least won't be flashy---no fireballs or magic missiles but plenty of charm person, hold person, blindness, etc. Spells must be learned from crumbling old tomes that blast sanity (and take weeks to learn a single spell) or are imparted directly into the caster's mind by evil demons or gods.
That all sounds fun.
Magic items are so rare as to be virtually unattainable, and you wouldn't want to advertise that you own one... not only will you be accused of devil worship, but evil people will seek you out to murder you and claim the item for themselves...
For gaming purposes, magic items can still be "virtually unattainable" while appearing regularly in the game -- and if their very appearance leads to drama ("evil people will seek you out to murder you"), so much the better.
As far as magic items feeling mundane, if you get rid of things like Wands of Cure Light Wounds for a handful of gold, you've gone a long way. A miracle like instant healing should feel miraculous.
Potions and charms (protective amulets) are an exception, though they will still be quite rare unless you happen to frequent the creepy hut of the old witch on the hill who can provide that which you desire... for a soul-damning price!
I love Oriental Adventures' notion of a Talisman in place of a Potion. Some low-level spells seem natural for one or the other.
I will also use Cthulhu's "classless" idea for characters:
2 feats, 8+Int mod skill points, choose offense or defense option for saves and base attack bonus...
I really only think you should use Investigator/Expert stats for wizards and priests. Conan should be a Fighter/Rogue, for instance. Also the Investigator/Expert is a bit underpowered compared to other classes, with no Special Abilities or Bonus Feats. I like to give Experts a Bonus Feat every other level (from a very short list of appropriate Feats, typically Skill Emphasis and a few others).
Anyone can learn spells but loses sanity for learning them, and again for casting them each time!
If wizards have all sorts of skills, this gives them something to do besides casting madness-inducing spells. Let Knowledge (Arcana) identify magic items, Spellcraft read scrolls, Knowledge (Nature) identify Animals and Magical Beasts, Heal treat wounds almost magically, etc.
Also, there will only be around 30,000 gp in circulation on the continent. Everything will cost silver pieces. Poverty is rampant! Hehe!
I don't see anything beyond a flavor detail in changing gold to silver. Adventurers should still capture what amounts to a king's ransom, only it'll be in silver, a smattering of gold, jewelry, silk, spices, art, etc.