The layout and lengths of the traces are important for proper functioning of the system. The goal is to minimize any signal distortion due to interference from other traces. The longer a trace is and/or the higher the signaling speed, the greater the crosstalk, which in turn means the more spacing that is necessary between the traces. Some traces must be limited to a maximum length to ensure signal integrity, such as those connected to the processor itself. All traces connected to a particular device must be matched in length within a certain tolerance.
A motherboard could use the highest quality components, and still be unreliable because the layout of the traces creates some signal integrity problems. The only way to know whether signal integrity is an issue is to measure it with an oscilloscope or with some specialized device built for this purpose. For overclockers, this could be even more important because the product may be running outside of the specifications that it was designed for and if the layout is not particularly good, reliability and stability could be a problem.
Various components attached to the motherboard require different voltages. The most common are 5V (BIOS chip, Real Time Clock, Keyboard controller, DRAM chips) and 3.3V (L2 Cache, chipset, SDRAM chips). Processors may require voltages from 3.5V to below 2V. Voltage spikes can easily damage all of these devices, so a regulator circuit is required.
The main power supply provides 5V directly to the motherboard, so voltage regulation is required for some devices. This is accomplished either with a plug-in module called a VRM, or by a voltage regulator circuit implemented in an integrated circuit and soldered onto the PCB. On a Pentium processor, two voltage regulators are generally required - one for the I/O voltage (3.3V) and the other for the processor (or Core) voltage.
In order to accommodate as many different processors as possible, the circuitry must allow for a range of voltages. This is typically accomplished with a set of resistors connected to a row of pins. When specific pins are jumpered, the circuit is routed through a particular resistor (or set of resistors) that provide the proper voltage to the processor. More recently, auto-detect circuits have been implemented that eliminate the need for jumpers.
Most new Pentium class processors are known as 'dual power plane' or 'dual voltage' processors, because their core voltage is different from that of the chipset and other devices (the I/O voltage). On the other hand, older Pentium class processors (and some newer ones, such as those from IDT) require 3.3V or 3.5V input. These are called 'single power plane' processors and motherboards that accept these must be sure to provide the proper voltage. To do this, the second voltage regulator is typically disconnected either via a jumper (VRE), or automatically through the use of a MOSFET device.