Decet is usually an impersonal verb, i.e., it has as its subject only a vague "it", as in the English saying "it is raining" (what is raining exactly? the clouds? the sky? no --- "it" is). As an impersonal verb it means "it is proper/fitting for". Vōs decēbit nihil dīcere. "It will be fitting for you to say nothing."
Here, it is not impersonal: it has a specific, stated subject, nec quidquam nisi argentum. To translate it literally: "For nothing except silver befits/adorns the table of a notable man."