I am arm'd Cassius probably is armed with a sword, but that's not what he's really referring to. He means that he's psychologically and spiritually prepared for whatever Caesar may try to do him. Cassius has just suggested that Casca may be a willing slave of Caesar, one who would inform Caesar of what Cassius has just said against him. Cassius is now saying that if that is the case, he's "armed" to suffer whatever Caesar may bring; after all, as he said just a minute ago, any true Roman is able to escape tyranny via suicide. All of this is part of Cassius' ploy to draw Casca into his conspiracy against Caesar. The ploy works; Casca immediately replies that he's not a tattle-tale, and he then says he'll do as much as anyone else to defeat Caesar.