Using MOSFET to control heating element using PWM and calculations
Your heating element at 50W rating driven at 24V will be drawing just a small bit over 2A. So the MOSFET that you linked to should be more than suitable to switch this load. 

There is no need to overdo the PWM switching frequency since the load is a thermal load with a long time constant. You could very comfortably be able to use a switching frequency of 500Hz to 2kHz which can easily be generated by the ATMega328. There is no reason to have to go into computing the maximum switching frequency of the MOSFET for this application.

In analyzing whether the ATMega328 pin can directly drive the MOSFET study Figure 1 graph of the MOSFET data sheet. With a gate drive of 4V (lower than the 5V that the MCU can actually provide) the 2A load line is way below the related VGS curve. This means that the MCU can comfortably drive this MOSFET for a 2A load. A MOSFET like this still has a pretty high gate capacitance and so it would be advisable to put a small values resistor in series between the MCU and the gate to limit the peak transient current some during switching.