Overclocking (451@851)... allows your burner to burn at a faster speed. No ripping speed enhancement.
Speedhacking... removes/relaxes burning speed restrictions based on media type. No effect on ripping, can be seen as a supplement to overclocking (OC lets your burner burn faster, SH lets you use that faster speed on more media types)
Readhacking... increases the reading speed of certain disc types in the 451/851 to 812S speeds. Potentially higher ripping speeds, depending on what media type you're dealing with.
thinks that maybe I should create a LiteOn FAQ for DVD burners, as things have indeed gotten more "complicated"...