The advantage, effectively, is better performance. But, only if it's a stable overclock. A faster clock means it can process more data in the same amount of time. An unstable overclock results in data being misprocessed, and artifacts occur (basically, weird s*** when you play games).
For nvidia cards, download Rivatuner and ATI Tool (search Google). Use Rivatuner to increase your core and memory clock, slowly. Then use ATI Tool to perform a never-ending stress test. Simply decide when it has run long enough, with no yellow dots (artifacts) in the window, for it to be considered stable for you (4 mins is enough for me). Then return to Rivatuner, and increase a bit more.
Im pretty sure the same software works for Ati cards, but am not too sure.