Sorry guys, I´m having the morning shift this week and it´s killing me. Of course you are correct as my A3640/25 has a 50 Mhz crystal, so in order to have it safely overclocked to 30 Mhz or more I need a 60 Mhz or 66 Mhz crystal ... in this case I would like the 64 and 66,66 Mhz crystals :-)
Anybody got an idea of how far you can overclock a Blizzard 4030/50 Mhz

With extra cooler of course :-D