Auto Insurance coverage -- How much Liability and Medical?

I got a couple of questions about about auto insurance coverage. Namely, what's the best amount of liability and medical coverage to select. 1) For the liability, I know it's wise to get the best liability that one can afford to cover your assets. But how about if one also gets umbrella coverage. For example, if one gets an umbrella coverage at a $2 million limit and the liability limit for the auto insuarance is $300,000 or $500,000, does it make sense to select the $500,000 or is it better to select the $300,000 limit since the umbrella coverage would kick in? 2) for medical, is this redundant coverage and not needed because wouldn't the liability portion also cover medical expenses? Or is it wise to get this, for example, if a passenger is not insured (no health insurance), so the medical part would help out in that case? Thanks.