The simplest way is to build up a string to execute:

$commandText = "INSERT DiskSpace VALUES ('" + $wmiObject.GetValue(0)["SystemName"] + "')"

$command = $conn.CreateCommand()

$command.CommandText = $commandText

$command.ExecuteNonQuery()

This way is not necessarily ideal as it is a little bit verbose, however, I would recommend building up the text as a separate string because you can always output it to the console before using it in anger:

Write-Output $commandText

Just say if you need more or if this is not what you were looking for.