Well one thing to remember is that the lesser Otherworldly Beings would have an easy save to keep your Sanity score up and though Psduenatural creatures are nothing to laugh at they would still have a rather low DC.

Also if you really want to do it like the Call of Cthulhu RPG then everyone except for Wizards and Psions would have an even easier time.

Personal I would run the Sanity score if I really wanted a HPL style game and I know my players would think it is an awsome rule. As well many would likely rather take Sanity damage then damamge to their spell casting scores.

As well if you are really converting the Sanity score to DnD rules then most Sanity damage is going to be tempary.

I'll work on getting up some rules so you all can know what i am thinking better and that way you can use them if you want.