I have used resistance heating as a dump load for quite a while. Presuming you only match the panels full output to the heating element with no controls, I think you will find it to be very inefficient in less than full sunlight. One solution would be to use the panels to charge up a capacitor bank and build a simple switching circuit to turn the heating element on at around the optimum voltage for the panels and off as the voltage drops too far below. In this way you could keep the panels operating nearer their optimum, which will give you more heat for your garage.
To echo OperaHouse, you will barely notice 500 watts in a 300 square foot or larger garage. Obviously insulating the structure would be a good idea as in general (thermal energy in - thermal energy out = accumulation in thermal energy) but of course in general, as the temperature increases inside relative to the outside, the heat transfer will also increase. Heat transfers by convection, conduction, and radiation only, but all three methods can increase with increased temperature differential between the inside and the outside.