Hi Simonsays
Just make sure that you take out a policy with "Worldwide" (including USA) cover for the maximum duration of your trip, remembering to include the transit time from when you leave your home to when you arrive back there, as no doubt you'll have a flight or two to catch as well which may well add an extra day or two. Include any extreme sports cover as a bolt-on if you think that you want to take part in these whilst away, otherwise you may not be covered in the event of an accident.
I always take my travel insurance from travelinsuranceweb.com and have always found they provide more than adequate cover at a really good price. We're not all the same though, so check that the policy meets your needs first.
Regards.