So, yeah. I know this is a low-end, outdated GPU. But I am, in fact, running an OEM PC, and I started out with a Radeon HD 7350, another shit card. So when my good friend offered to give me his GT 520, I sure as hell wasn't going to say no. I noticed immediate tangible performance increases. But then I wanted to push it. I know, I'm not going to get much out of this, but I still would like to overclock it. I am new to OCing hardware, and would like to know some safe numbers that I can overclock the 520 to.
Overclock little by little, monitor the temps and get the highest clocks that are stable.
I would get a utility like MSI Afterburner, and try pushing up the core clock slowly, Bit by bit, running "ATITool" or "Furmark" after each bump to test for any artifacts. Once you find the max Core clock you can do, Return it to normal, and do the same testing with the Memory clock. Then leave the memory clock up, and raise the core clock BACK to the highest working clock you had before. Then test it. If there are no artifacts, you WIN :D. If there are, bump the memory or core clocks down just slightly, and retest.
I would deffinately not try messing with the voltages, unless you know what you are doing. As long as you dont mess with Voltages, you shouldnt break anything.