What 520W PSU do you have? Also, what CPU and graphics card are you using. The specific components are far more important than how many of them there are, you could run 4 5570s off of a good 520W unit if you wanted to, but only one higher end GPU.
If all you want to do is add another fan you should have more than enough power, they usually draw less than half an amp.
Core i7 9xx require 130W, Core i7 8xx/2xxx require 95W
Core i3, 75W
Core i5, 95W
Core 2 Duo 65W - 95W
Core 2 Quad, 95W
AGP graphics card, 50W
Graphics card w/ 8pin and 6pin PCI E, 300W
Graphics card w/ 6pin and 6pin PCI E, 225W
Average motherboard, 35W
High end motherboard, 50W
Two sticks of DDR3, 6W (Two sticks DDR2, 8W)
Three sticks DDR3, 9W
Each fan, 6W
Each optical drive, 10W
Old hard drives, 15W
New hard drives, 10W (Also SSD)
Add all your components up, and divide the number of watts they use by the number of watts your PSU has to get a percentage. You pretty much want this percentage around 50%-75% if you plan on adding more stuff into your machine!
Your Celeron is using 65W or less, your GPU 50W, your HDDs 40W altogether, your optical drives 20W, your motherboard 35W or less and your fans 24W... total of 234W, which is 45% of your PSU's total output, you're good to go!
Your PSU might be sufficient enuf for the current hardware, but that is just when they are working at the normal levels. So try not to consume all the watt as some hardware , such as CPU or Graphic card tend to overdraw when under pressure. So do not fill the bottle upto the brim.