The use of very high dot-clocks has a REAL negative effect on the performance of the boards, due to its limited 80Mbit/sec, higher dot clocks limit its ability to draw data into the framebuffer. Thus expect better performance of a 72Mhz based mode than on a 78Mhz based one (for example) where more bandwidth is required for screen refresh.
It does not make much sense to use the highest clock (78 MHz) for 1024x768 at 76 Hz on a OTI087; the card will almost come to a standstill. A 72 MHz dot clock results in 70 Hz which should be acceptable. If you have a monitor that supports 1024x768 at 76 Hz with a 78 MHz dot clock, a standard OTI087 based card is a poor match anyway.