I agree with DamageX. You can't just change the crystal and CPU to double the speed of the card. The rest of the logic components on the card won't be able to take that type of timing.
IF (and that seems unlikely) the 50mhz board unit contains all the same components and layout as the 25mhz board unit, there is probably a jumper/trace difference that halves the clock rate for the rest of the components (or sends the half-refresh signal to the CPU, thus doubling it).
In other words, running a 2x multiplier 25mhz clock for the 50mhz version vs. a normal 1x multiplier 25mhz clock on the 25mhz model.
