Mr. McDowell,

Regarding the issue of when high frequency becomes an issue, don't confuse clock speed with signal frequency. Think of frequency as dv/dt, not clock speed. You can be running a clock speed of 1 kHz and still have frequency problems if the clock signal switches states fast enough.
Most "high speed" problems are related to reflected signals, poorly matched loads and sources, as well as stray capacitances, inductances and resistances. And of course, at high speeds every component in your circuit (including the conductor) is actually equivalent to a network of all three.
Think of it this way. Figure that your signal is going to propogate at about 6 or 8 inches per nanosecond. Now, how fast does your signal switch states, and how long is your conductor? If your signal transition is 1 ns, and your conductor is 6 inches long or longer, then there will be a time when the source is high and the load is still low. There's more going on than can be explained in a message board posting, but when the signal starts reflecting it can cause spurious switching, EMI radiation, etc..
You'll need to read up on the subject if you're interested. The point is that the issue is transition time, or dv/dt, NOT clock speed. So don't be lured into the myth that slow digital circuits aren't sensitive to high speed errors. If you use modern components they are apt to have very fast transition times, and consequently can exhibit "high speed" problems if your conductor is long enough. I concern myself with such matters anytime my conductor is more than 1/4 to 1/2 the critical length (where the critical length is the distance the signal will propogate in the time it takes to change states).
Good Luck.