Limit for trans-Atlantic tends to be 2 pieces, up to 28 kg each. If your internal flights in the US are on the same ticket then that baggage allowance carries through for the internal flights. Otherwise check the airline's site for details of allowances/charges for baggage on domestic US flights. Take 2 bags and underfill rather than one full one because the excess baggage charges for overweight bags are pricey.
Annual insurance policies tend to be the best value. Ours is currently with ACE through AIB and we pay about 70 euro a year for worldwide cover for both of us (we have the private healthcare plan which is discounted because our health insurance gives us a certain level of cover abroad anyway).
For insurance to the US the main things to make sure you have are medical cover in excess of $2,000,000 and personal liability of at least $500,000 IMO