What sort of status does she currently have? TN? H1B? When does it expire?
Your problem may lie with getting married in Canada. If she leaves the US, then gets married to you, then tries to re-enter the US the could deny her entry and then bar her from coming back until she gets a spouse visa (which currenlty take about a year to process).
I'd reconsider the wedding in Canada until *after* the process is complete. For now just have a civil service at the courthouse or something.
For marriage based immigration to the US info go to
http://www.visajourney.com ALL the info you need is there, we're primarily a traveller's forum here, not immigration.