Raising rent - new purchase

[Steve Yoder]

I'm purchasing a property (over 20 units) with the rents extremely low ($650/month average) for the area.  Average rents for the area are $800/month and the building is in a good location and is in good condition.  I'd like to get rents close to $800/month asap without loosing a lot of the tenants.  Most tenants are month-to-month. 

Do I raise rents a bigger amount the first year ($100) and a smaller amount ($40-50) the 2nd year or raise it where they should be the 1st year ($150)?   What have you found the be the best way to raise the rents without incurring a lot of vacancies?