One of the most attractive things about PCs is their upgradeability. To add extra functionality to your machine, you simply need to drop in a suitable card.
There have been a number of different expansion slot interfaces built into the motherboard for expansion cards such as sound cards, graphic cards, modems and network cards. There are four main types of expansion slots you'll find in a new motherboard - the now very rare ISA slot, the PCI slot, the AGP slot and the PCIe slot (PCIe slots are, incidentally further broken down into various speeds).
A given motherboard is going to have a certain number of expansion slots of each type it supports. The specifications may tell you, for instance, that the motherboard has three PCI slots, two 1X PCIe slot and one 16X PCIe slot.
The key question you'll be asking is whether to get a PCIe or AGP motherboard. The two are (not technically, but in practice) mutually exclusive. PCIe is newer, faster and more flexible, but if you have an AGP graphics card that you really want to use, you will need to buy an AGP motherboard.
Originally developed by Intel, the Peripheral Component Interconnect (PCI) is a local bus channel used to transfer data to (input) and from (output) a computer and to or from a peripheral device. Most PCs have a PCI bus that is usually implemented at 32-bits and provides a 33MHz clock speed with a throughput rate of 133MBps, although you can get PCIs that transmit 64-bits in an expanded implementation.
PCI cards come in two lengths. The full-size PCI is 312mm long, and short PCIs range from 119mm to 167mm. Most current PCI cards are half-sized or smaller.
Nearly all motherboards, even PCIe motherboards, feature PCI slots, since most add-in cards still use the PCI standard. Usually the minimum is three, but motherboard manufacturers have extended this up to as many as six slots. This is important, because if you want to use a lot of expansion cards, you will need as many PCI slots as you can get.
Accelerated Graphics Port (AGP)
In addition to PCI slots, many motherboards will also feature an accelerated graphics port (AGP) slot. The AGP slot is exclusively for graphics cards. It is being phased out with the introduction of PCIe.
The AGP bus is a dedicated high-speed port just for the graphics controller. It offers superior bandwidth capacity to the PCI bus, meaning data can be transferred in bulk and at a faster speed. This is because graphics operations using an AGP slot do not have to share bus bandwidth with other peripherals, like they would using the PCI bus. It also means the PCI bus can run more quickly, as it is free to serve other devices, such as the hard disk, sound card, modem and network cards.
All new AGP ports will be '8X', which means that they run eight times as fast as the original AGP ports (you need an 8X AGP port to run an 8X AGP card). An 8X AGP port runs at roughly the same speed as an 8X PCIe port.
A new and increasingly common standard, PCI Express (PCIe) was designed to replace both AGP and PCI. In the next few iterations of motherboard technology, we can expect to see both AGP and PCI totally phased out in lieu of PCIe. For the time being, however, many PCIe systems still have legacy PCI slots, for compatibility with the vast array of PCI expansion cards on the market.
PCIe slots come in a variety of different speeds - 1X, 2X, 4X, 8X, 16X and 32X (roughly corresponding to the AGP speed levels). A motherboard may have two 1X slots and one 16X slot, for instance. The slots for different speeds are physically different - the 1X slots are quite short - barely an inch, while successive levels get progressively longer.
One of the more interesting design aspects of PCIe is that you can use cards that are smaller than the slots. You can put a 4X card in a 16X slot, for instance, or a 1X card in a 4X slot. It won't fill the slot to the end, but it should work just fine.
In a typical motherboard configuration, the longest slot (usually 16X, but sometimes 8X) is there to be used for the graphics card. Most PCIe graphics cards you buy will be of the 16X variety. The shorter 1X and 4X slots are there for less-bandwidth intensive devices like sound cards and TV tuner cards.
One some motherboards, you may find more than one long (16x) slot. That's because the motherboard has been designed with SLI (scalable link interface) configurations in mind. With some graphics cards, most notably Nvidia graphics cards, you can actually put two graphics cards in the one system and they will work together to render a scene, each taking half the screen. To do this, you need two slots capable of supporting the graphics cards. In practice, that means having a motherboard with two 16X PCIe slots. SLI is not possible with AGP, which is limited to one slot per system.
The predecessor to PCI was the Industry Standard Architecture (ISA), a standard bus architecture. ISA allows 16-bits at a time to flow between the motherboard circuitry and an expansion slot card and its associated devices.
ISA and PCI were easily distinguishable: ISA slots were black, and PCI white. If you have a look inside a modern PC, however, you won't find any black ISA slots, as the standard has been phased out.