28 MHz should be possible, 33 MHz are critical. It`s not the processor that makes problems it`s all the other parts that are on the board.
Anyway even if you could successfully clock an A3640 at 40 MHz it would still be outrun even by an Apollo4040 @ 25MHz.
You can`t win much with rising the clockspeed on an A3640 as the design (especially the lack of own RAM) sucks-
