I've tried to overclock two A3640 boards to 33MHz but was unsuccessful.
Don't quote me on this, but I think one of the limiting factors here is that the A3640 clock also takes control of some of the motherboard timing circuits. Thus, when you overclock the card, you also are overclocking the motherboard. Other accelerator cards besides the A3640 seem to leave the motherboard clock alone. At least this is the impression I get by the jumper configurations that need to be set on my A3000 motherboard when switching between an A3640 and a WarpEngine 4040.
You're best bet is to just look for another accelerator board if you need higher speeds.
