It's been a while since I've rented a car in the US, but from what I remember, I just basically asked whats the minimum level of insurance I should go for? I think it was that Collision Damage Waiver and insurance to cover the 3rd party's car in the event of an accident being my fault.
This was in April 04 so maybe things have changed since then and there's more mandatory requirements etc.
If you do have a motor policy here, I know you can pay extra to allow coverage in other countries, I'm not entirely sure if it'll cover in the USA, you may need to find out but I'm skeptical.
The good thing with the 'rental' car insruance is that even IF you do have a accident or a claim (theft etc) , then the 'record' of it doesn't show up on your personal policy - if you have one of course.
Seeing as you've said yo dont have a policy here, I think you're only option will be the insurance provided by the rental companies. I think Credit Card insurance doesn't cover driving car's abroad and 'motor vehicle insurance' as such, although maybe it's a benefit for certain banks and credit card members/accounts?
I think if you add up all the 'extra insurances' the price of the rental goes up substantially, but cut out the ones you wont need to achieve a balace for the level of risk you're willing to accept (for me I went for the absolute minimum, but included that CDV charge and I was happy with the costs overall)
Hope this kinda helps!
Cheers! DtM! West London & Slough UK!