Hey Nichole,
You can house certain pieces of information in Marketo through the use of Custom fields. As long as you do not sync these to your CRM, then the data will only be viewable in Mkto regardless of whether the Lead has been synced or not. Depending on what you are looking to keep out of your CRM, this could be a lot of field creation.
And yes, Sales Insight requires the Lead to be in the CRM. Sales Insight allows the rep to see the Mkto information via your CRM. Can't have Sales Insight without the CRM.