Hi in the past i have overclocked a couple of 5200's on old g31 boards. It has been a long while so dont take my word as chuck norrises or anything but i can give you a basic outline. They are not the best boards for overclocking but it can be done pretty easily. I used to get an easily stable 3.2ghz, and have gone up to 3.7 once (not stable on this board)
There should be an advanced header in your bios which includes options for CPU FSB Speed, DRAM Speed and so on. In that their should be a tab for your cpu where you can adjust the FSB frequency (base frequency before the multiplier is applied). Check to see if you multiplier is at its max (12.5) and all you have to do from their is adjust the FSb frequency from its stock 200MHZ...