I've never seen it written in the ARCI form (except here), it is RACI, and it's pronounced (ray cee).
I have to say though that it may be possible that some people will use it as ARCI instead of RACI, here's why... If you think about it, the letters in RACI start from the highest involvement on the project (Responsible) to the lowest one (Informed). But the question is, does the word "Accountable" imply a lower level of involvement than "Responsible". Some people probably think it does, while I (and many others who think that RACI is the correct word) think otherwise: while you may be accountable for the project, you don't have to be as involved when you are responsible. Of course, accountability means more burden, but it doesn't necessarily mean more involvement.
It is RACI!