fido compatible phone HELP

Hi I ve been looking a previous postings on this subject. I am actually a prepaid client with FIDO. I actually want to swicth cell phone and buy a new one from Europe ( which are way better looking). HOwever I am not sure of the prerequisite I should look for in order to buy a new cell phone. Will I be able with my sim card to just place it in the new cell phone in order to make it work? OR should I buy a unlocked cell phone? I am actually looking in a tri-band

THanks for you help
Any GSM phone that is unlocked with 850 and/or 1900 mhz frequencies will work.