greetings and welcome.
you maybe confusing the "dual quad phone" for
Dual sim card
Quad band phone.
The dual sim part, if it is one, allows the user to install 2 sim cards. So, if you happen to be dual country type of person, you can have a sim (phone number) for each country. They are nifty, but 2 woudlnt be enough for me.
you can use a sim from one country in a different country. the gotch ya, is that there will be roaming charges for data and calls will cost more.
here is what i would do if it was me.
If i planned on traveling frequently and want to use it in the usa, i would buy a local sim here. That way any good friends you have wont have to dial and be charged for international phone call to you.
just a comment and i dont work or will benefit from this is that T-Mobile offers a nice monthly package including international. so check it out if its good for you. Also, check out your current provide what they can offer/match or beat.
i dont think a european sim is needed unless your USA phone has horrendus international rates. however, if your current plan screws you royally, then i would look over there. check out carphonewarehouse.co.uk. they have some carriers listed along with their charges.
whats nice about over there is that i can get a PAYG card/number and not get shafted for its use. If i want to use internet i can buy it for a day, days or a week at a time and its not that bad. overhere, i have to purchase one of the more expensive packages to get internet. just so you know, if you do buy a sim from over there, VERIFY you can add minutes.