if you game, OCing is definitely worth it. GPU OCing is the best, bringing nearly 100% performance scaling. my 15% overclocked 6600GT lets me play HL2 at 1 resolution higher, or lets me turn on AA or AF. For a midrange card, memory speed is the most important - since the main limiting factor in a 6600GT is its 128-bit memory bus. If you overclock the memory to, say, 1200MHz (from 1000MHz), then you are relatively close to the bandwidth of a 6800 non-ultra. GPU speed brings gains too, but not as much as memory. On the high end, memory and GPU speed are pretty much equally important.
CPU overclocking doesn't bring quite as much. I OC'ed 41%, but i didn't get anywhere near 41% performance boost. maybe 15-20%, still worth it, but not as much so. For other stuff, though, like media encoding, etc., it's more of a linear scaling.