io farei così:

<?
for($i=0,$i=sizeof($_POST['Codice']);$i<$dim;$i++)
{

if ($_POST['Quantita'][$i]!=0){
$risultato =mysql_query("SELECT Codice, Titolo, Voce, Var1, Var2, Unitadimisura, Prezzo, Quantita,(Prezzo * '".$_POST['Quantita'][$i]."') AS Totale FROM opere_vetraio_materiali where Codice='$cod'")
or die("Query non valida".mysql_error());

while ($riga = mysql_fetch_object($risultato))
{
?><tr><td height="75"><input name="Codice[]" size=15 type="text" value="<? echo $_POST['Codice'][$i];?> " readonly>
</td><?
?><td height="75"><input name="Titolo" type="text" size=15 value="<? echo $riga->Titolo;?>" readonly>
</td><?
?><td height="75"><textarea name="Voce" readonly><? echo $riga->Voce;?></textarea>
</td><?
?><td height="75"><textarea name="Var1" readonly><? echo $riga->Var1;?></textarea>
</td><?
?><td height="75"><textarea name="Var2" readonly><? echo $riga->Var2;?></textarea>
</td><?
?><td height="75"><input name="Unitadimisura" size=8 type="text" value="<? echo $riga->Unitadimisura;?>" readonly>
</td><?
?><td height="75"><input name="Prezzo" size=10 type="text" value="<? echo $riga->Prezzo;?>" readonly>
</td><?
?><td height="75"><input name="Quantita[]" size=6 type="text" value="<? echo $_POST['Quantita'][$i].' ';?> ">
</td><?
?><td height="75"><input name="Totale" size=10 type="text" value="<? echo $riga->Totale;?>" readonly> </td></tr><?
}

}

}
?>