Be careful. Test the phone if you can before handing over the cash. Some models ie. Bold 9700 from Wind Mobile will not work on other carriers 3G network. It only works using EDGE on Fido, Rogers, Bell, or Telus.
Calls work fine, but the download speeds will be limited. If it's a Rogers branded handset, you will be fine. Most other carriers as well.
Yes because fido uses 850 and 1900 mhz in both GSM and UMTS networks. So if your going to buy a blackberry 9700, check the specs because the wind one operates on slightly different frequencies in the UMTS band, which is what you should look for preferably if you want high speed 3G.