You could certainly estimate the clock frequency in advance. The things that are going to cause trouble are transmission line effects (ie pulse length no longer >> wire length), and the usual digital logic critical path constraints: wire and gate delays; clock skew and drive strength; fanout constraints.
More decoupling doesn't help you once you have enough to prevent crosstalk through the power supply. Star-routing the power and ground may be a good idea.
I've done 20MHz. Coincidentally this is the limit of cheap scopes. This page claims 33MHz is achievable, and gives a good explanation of why: http://www.sigcon.com/Pubs/news/2_8.htm
For IC availability, check your local Digikey. Note that 68k is still being manufactured as "Coldfire" by Freescale (with slight backwards incompatibility).
You could certainly estimate the clock frequency in advance. The things that are going to cause trouble are transmission line effects (ie pulse length no longer >> wire length), and the usual digital logic critical path constraints: wire and gate delays; clock skew and drive strength; fanout constraints.
More decoupling doesn't help you once you have enough to prevent crosstalk through the power supply. Star-routing the power and ground may be a good idea.
I've done 20MHz. Coincidentally this is the limit of cheap scopes. This page claims 33MHz is achievable, and gives a good explanation of why: http://www.sigcon.com/Pubs/news/2_8.htm
Some good tips: https://www.pjrc.com/tech/8051/dev-board-wirewrap.html
Reccomendation to use the kind of board with built-in ground plane (sounds good if you can find it): http://www.williamson-labs.com/prototype-lt.htm
For IC availability, check your local Digikey. Note that 68k is still being manufactured as "Coldfire" by Freescale (with slight backwards incompatibility).