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This is the very heart of a modern PC. Not only will it determine the flexibility of the PC but more importantly it's stability. 

The  first thing I had to decide which processor I was going to use. After my first experience with none Intel chips I was reluctant to try another but about 14 months ago a colleague at work wanted 3 identical gaming PCs building and asked me to spec them. After some research I suggested an AMD XP1800 partnered with a K7S5A SIS mainboard. These PCs turned out to offer brilliant performance for the relatively cheap costs so I built one for myself. By October 2003 I decided that a faster processor would now be available at reasonable cost. Initially I looked at the XP2600+ but found out that the new Barton cored XP2500+ was cheaper. After reading a few reviews I realized this was the sweet spot in terms of price and performance. The main difference between the new 'Barton' CPU and the existing 'Thoroughbred' is the increase in level cache (L2) from 256Kb to 512Kb. This will give an increase in performance where the application code calls for the same instructions repeatedly, any such instructions are stored locally (cached) just before execution. This is much faster than the CPU having to access the main memory, not only does main memory run at a much slower speed than the CPU but also main memory is in a physically different location. Both these aspects incur a speed penalty, the faster the CPU gets the harder it is to keep it constantly fed with the data it requires. This increase in cache size and clock speed does amount to a sizeable jump in performance that's easy to see, for example the encoding of MP3s jumped from 24x real-time to 32x real-time.

The next decision was which mainboard should I choose. This is not always an easy job, especially if you are trying to anticipate what your needs will be until your next major upgrade. The trend at the moment in the industry seems to be to provide either very basic facilities - such as the ECS K7S5A or all the bells and whistles such as the ASUS A7N8X Deluxe. With this comes another problem, as an architecture matures the quality manufacturers tend to move away from the basic stuff to the more profitable high end. This can mean that the only basic boards available are made by unknown companies with poor quality control and support, this can make buying a basic board a bit of a lottery. When I bought the ECS K7S5A board, the chipset on which it was based had only just become available and so was only offered by a few reputable companies, I've built 5 PCs using this board and never had any issues with stability which goes some way to proving my point. This time however I decided to spend more and get a quality board that was not only fast but also had all the major interfaces and facilities that I would need for the next year or so. When choosing a mainboard there may be several chipsets available that will support your chosen CPU, manufacturers such as ASUS usually offer mainboards based on several different chipsets - such as Nvidia, VIA, SIS, etc. The chipset is the heart and soul of the board, it will to a large extent determine the ultimate overall performance of the PC and determine which basic facilities are available. Manufacturers can add extra chips to the board to give extra functionality but this adds to the cost, a chipset that offers all you need without extra chips will be cheaper when turned into a mainboard. However usually there's some facility offered by one chipset that's not offered by another and vice versa. 

Below - I finally decided on a Nforce2 mainboard made by ASUS, the A7N8X Deluxe.

Here is why - 

Performance - the NForce2 had at the time a unique dual memory architecture (Twinbank) that allowed it to access two banks of memory at the same time with the resulting performance increase. This won't translate into double the memory performance but somewhere in the region of around 10-20% increase in memory bandwidth. Peak bandwidth up to 6.4GB/s

Below - the 2 blue sockets are the one to use if you wish to utilise the Twinbank technology. In this shot a single DDR DIMM is fitted in the standard slot.

Sound - Nvidia was the first to integrate a sound chip that was powerful enough to encode Dolby Digital 5.1 in real time, lots of sound chips can decode it real time but that's far easier. This seemed like a good facility to have, I can pipe the audio output digitally to my Dolby receiver which can then decode the signal, this should ensure good quality with a lack of interference.

USB & Firewire - the Nforce2 chipset supports USB 2 providing up to 6 ports for connecting to. 4 of these are built into the ATX main back plate whilst another 2 are provided using a slot in back plate and 'header' connections. USB 2 provides up to 480Mbps. 2 Firewire (AKA IEEE1394) ports are provided, 1x full size and 1x mini. Both these are added using a slot in back plate and headers. Firewire provides up to 400Mbps. USB is by far the most common interface of the two and which I use to connect up my Sony Cybershot camera and MP3 player. However I still wanted the facility of Firewire as it is the interface of choice for digital video editing.

Above - Here it's easy to see the 2 LAN ports and the extra sockets for the audio including the co-axial digital out (single RCA socket).

Networking - the ASUS board offers dual LAN ports, although I don't currently use them this facility would allow the PC to act as a server/firewall in future.

SATA - this facility isn't a native function of the Nforce chipset but is provided by extra chips on the mainboard. It's a good improvement over the existing PATA (parallel ATA) in so much as the connecting cables are much thinner, flexible and can according to the specification of the interface use longer cables. Other abilities such as 'hot swapping' (connecting/disconnecting with the power on) are also supposed to be possible. The speed of the actual interface isn't significantly faster than the existing PATA interface, 150MBps versus 133MBps. But it is much tidier in PC cases that solely use this technology. Another benefit of moving over to SATA hard disk drives is that the unused PATA interfaces can be used for extra CD/DVD/ZIP devices. Which in the case of the ASUS board allows for a total of 6 devices.