AMD is specific about which thermal compounds it recommends. While thermal pads are recommended for lidless CPUs like Athlon, Athlon XP and Duron chips, lidded processors like the Athlon 64 should have a thin layer of paste applied to the lid prior to fitting the heatsink to it.
If you purchased the boxed version of the Athlon 64, and it came with the cooling system supplied, the thermal compound provided will be perfectly adequate, especially as you don’t intend to make excessive demands of your CPU.
If you ever did want to increase the demands of your CPU, familiarise yourself with heatsink removal and cleaning. You should never mix different thermal compounds, so you would also need to clean both CPU and heatsink before applying new thermal paste. AMD provides detailed videos for both processes at www.amd.com/uk/HPBodySystemComponent/.