Creating small heater
No, you don't need an isolation transformer provided the low voltage power supply, relay and temperature sensor are isolated from the resistor and mains circuit. If any of those isolation barriers is not present or fails, even for an instant, bad things may happen (such as a shock, fire, destruction of your computer, etc.)  
The power the resistor can dissipate is given in the datasheet. 




If you are using it to heat something, say to 100°C, then you can dissipate about 80% of 50W or 40W. The actual power will depend on how insulated the resistor is as well as the ambient temperature (it's the air immediately around the resistor that matters). 


That voltage rating is a power dissipation limit. There is also a 1000V test. That part should be fine on mains voltage barring any huge surges.