Drivers: It depends on the manufacturer. Apple OEM and most Sonnet ZIFs don't need drivers at all, they're totally plug-and-play. The only drivers one might need is the AltiVec code for the G4s, installed automatically with new installations of the OS and software, or installed specially by the manufacturer's utilities.
PowerLogix, XLR8, Newer, and others can require special drivers to enable their L2 caches, in both OS 9 and X. They're usually available from the manufacturer, or from the Internet at large.
As for a PC heatsink, yes, some will fit. However, one must be careful that the heatsink's retainer clip doesn't fit too tight and is installed with the center of pressure over the die of the chip; it's possible to crush the die on those, so one must be careful.
Also, there must be adequate clearance in the case. For example, the G3DT has only about an inch between its stock heatsink and the top of the case. The G3MT has about 1/2" more clearance, and the G3AIO has the most room of all.
B&W G3s and the Sawtooth G4 have less than an inch between their stock heatsinks and the optical/Zip drive setup in their cases, so it's not really ideal to install a larger heatsink in one of those.