Everything in a Plus runs off a single crystal which was chosen for compatibility with the SCC chip and standard baud rates. If you change the crystal, the Mac may beep and start, but everything (disk, serial ports, video, sound) will be off. Video, sound, and disk speed are DMA interleaved with the CPU, so all have to mesh. Horizontal sweep is 22.25KHz, much higher than NTSC video at 15.75KHz, which is one reason flyback transformers run hot. Frame rate is very high to avoid flicker and eye fatigue (Has to pass Andy Hertzfeld's potato chip test, where you stand across the room and eat a chip and see whether the screen flickers noticeably). Wikipedia has a good description of the Mac circuitry.

The Apple II uses a 14.31818MHz crystal (4x color burst) which gives it the ability to generate pseudo-color video on a TV.