I was struck by the HCSB translation of Matthew 5:
But I tell you, everyone who is angry with his brother will be subject to judgment. And whoever says to his brother, 'Fool!' will be subject to the • Sanhedrin. But whoever says, 'You moron!' will be subject to • hellfire.
There's a footnote after "Fool!" that says:
Lit Raca, an Aram term of abuse similar to "airhead"
On the one hand, I don't generally approve of translating words that in the Greek are foreign (in this case Aramaic) into English translations as English. It's not a Greek word, so translating it into English should involve keeping the foreign word as a foreign word, other things being equal. But Matthew's readers would have know the word, or he wouldn't have used it. English-speakers generally don't. So other things might not be equal in this case.
That issue aside, I think the HCSB has it right with "moron" and "airhead". Those words have much more force than the typical "fool" used in this passage. The downside is that Jesus may well have intended a connection with the fool of Proverbs, who usually is called a fool in English translations. But the English "fool" doesn't exactly capture that either. The term "moron" really does capture the anger element Jesus is getting at, and "airhead" isn't bad for "Raca".
It would be fun to ask people where the word "moron" is in the Bible and see what they come up with. It would be interesting seeing how certain people respond to Jesus saying that calling someone a moron is sufficient for deserving to burn in hell (I'm thinking of people who see Jesus as all mercy as a revision from the wrath of God in the Old Testament). All it takes is calling someone a moron. I know the Sermon on the Mount has pretty high standards, but think about that for a little bit.