You'll want to investigate this fairly carefully - the "cost" of sticking with your phone might be pretty high...

For example, T-Mobile to go if you only get a SIM charges something like $50 for the SIM with $10 "value" on it. however, they often offer you a special "package" with a phone + sim for $30 w/ $10 value. In my opinion, you'd be better off buying the package with the phone/sim for $30 - and then switch the SIM from your "new" phone to your "old" phone.

Regarding Canada - if you're going to be calling Canada frequently and roaming in Canada frequently (say more than a few times a year) - you really may be much better off getting into a "post-paid" contract with a carrier. They generally offer North America plans that include roaming in Canada (i.e. no extra charge) and often free or heavily discounted rates to call to Canada.

Again - you probably won't have to switch phones - and if you do you can get them to transfer your contacts from old to new phone generally for free.

Good luck.