Raca means "empty headed," very similar to how we use "fool" today. Jesus also uses moros in that verse, which is the root of moron. While we normally need to take care not to commit the root fallacy, this one does mean the same thing.
The word used in Hebrew is nabal which has more to do with consistently making bad moral choices. Brown, Driver, Briggs starts its entry with "foolish, senseless, esp. of the man who has no perception of ethical and religious claims, and with collat. idea of ignoble, disgraceful;" The Psalm is not referring to atheists as we think of them (the modern atheist would be unheard of in the ancient world), but of moral reprobates.
Some other interesting uses of nabal in the Old Testament.
In Is 32:5 it is opposite to nadib which means noble minded. There's a nice Hebrew word-play here, too. "No longer will the nabal be called nadib.
Pr 17:17 says that the nabal's arrogant speech only makes his faults more conspicuous.
Job 30:8 teaches that these "ignoble men" come to a bad end.
Jer 17:11 says that those who earn their fortune unjustly will lose it and prove they are nabal.
Dt 22:21 says that women who play the harlot have committed an act of nabal. That is definetely a moral issue not merely an empty headed act.
The Septuagint uses both moros (foolish) and aphron (ignorant) to translate nabal.
It should be noted that Jesus is warning us not to call particular people nabal (I think it more likely he was using that word in the second phrase and not the first) while the Psalm is pointing out that certain people have the characteristics of the nabal.