I started prototyping the program counter, and ran into a bit of a jam. The 74161 synchronous counters that I ordered...looks like I inadvertently picked up the "S" version (74S161) which is rated at 80Mhz and draws a whopping 475mW at 5V. With four of them (for 16 bits) in parallel, I measured 2 ohms across the power leads:
5/2 = 2.5A
Yikes! My little 1A 7805 simply can't drive them. What I *wanted* was the 74LS161, which is rated at "only" 30Mhz, but uses a reasonable 35mW at full load.
I'm also considering running an unregulated 12V rail to each sub-board, so that if a sub-board needs more power than can be provided by the clock/control unit, a 12V rail is available for a 7805 to bolster the power capacity.