First off, 500w psu is more then enough for that system, that system minus the graphics card will draw around 60-70w, the graphics card can draw 75w max.
If you are using amd catalyst control center to do the overclocking there is really no downside to it since you cant adjust the voltages.
Increasing the clocks (for the core first, do the memory after) by 15mhz then check to see if its stable.
I recommend using unigine valley for stress testing, run it for 10-15mins if you see no artifacts or no crashes after that time increase the clocks by 15 then repeat.
Remember to check temps while overclocking, gpu-z is a good program to monitor temps.
Once you find a clock speed that is unstable reduce the clock by 5mhz each time until you find the highest stable clock.
Once you've found the stable clock i usually run valley again for 30-40mins just to make sure the overclock is solid.
Repeat for overclocking the memory, unless your card has a temp probe for the memory you wont have to monitor the temps so closely.
As for memory oc'ing not offering any advantage, that will differ from card to card, when i was running a hd7950 oc'ing the mem offered quite a considerate increase in performance, my r9 290 on the other hand gain next to nothing from oc'ing the memory.
DDR3 is a huge bottleneck for a graphics card, as it's a low end card i'm unsure how a mem oc will effect performance.
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