FET atmega168 - transistor needed?
Power MOSFETs normally have relatively high Vth, which requires voltage higher than MCU will output. Also the gate capacitance will normally require high current (sometimes even 5A!) for like tens of nanoseconds each PWM cycle, which is way beyond what MCU can do. And of course the high side MOSFET's source voltage rises to Vbus, so to drive it's gate you really need some driver that will rise with the MOSFET.

Take a look at Silabs isolated gate drivers. I use them for the last couple of years, they are just perfect.

http://www.silabs.com/products/power/isodrivers/Pages/default.aspx