RAM is one of the most important components in a computer. It determines how fast your system will run when operational including start up time, multi tasking performance and virtualisation (if you're into it). This is where the computer temporarily stores data for quick access. If you run out of RAM, the computer uses hard drive space (virtual memory) instead to compensate for the lack of temporary storage. Hard drives are much slower than the memory used in your RAM so your computer can become extremely unresponsive if you don't have enough. Because of its importance, this is one part you need to consider carefully when you build your computer.
Triple Channel Kits
The latest DDR3 RAM comes in matched sets (called kits). These matched pairs go into triple channel (corresponding colour) RAM slots which make the transfer speeds triple what it would have been if you had just used one stick of RAM. These triple channels are recommended to get the maximum performance from your system. It is recommended that the RAM be the same make, speed and capacity. Since if you have one stick of slower RAM and one stick of faster RAM then the speed of your faster RAM will be slowed down to match that of the slower stick of RAM, hence you are getting poorer performance. Whether you can take advantage of triple channel DDR3 depends on your motherboard. Most newer boards will but some cheaper ones still have dual channel DDR3. DDR2 is now old and probably more expensive than DDR3 in most places.
How much is too much?
The amount of RAM you need differs. With the release of Windows 7, you need at least 4GB of RAM to run aero and decent games but with triple channel, you probably want to buy a 6GB kit to take advantage of triple channel. If you do a lot of photo editing and intense gaming, you may want to consider 12GB of RAM or possibly even 12GB. Today, RAM is quite cheap. You can get some decent 6GB memory for about AUD$80.
Remember that in order to install either 6GB or 12GB of RAM you must have a 64-bit operating system. 64-bit is now the same price as 32-bit and there is quite a lot of driver support for it with Windows 7. There is no reason to opt for the 32-bit version any more as it really limits the upgrade potential of your computer. It limits your computer to a maximum of 4GB of memory including video and sound memory. So if for example you had 4GB of RAM installed and a 1GB graphics card, Windows will only recognise 3GB of RAM in order to give the video card its 1GB of memory. This can have a significant impact on your computer, not to mention it is wasting a whole extra gigabyte of RAM which could speed up your system. One problem which I have fond with 64-bit Windows is that it doesn't support many camera RAW codecs. Canon has not developed any RAW codecs for 64-bit Windows but third party options like Fast Picture Viewer are available for keen photographers.
Memory Latency, Is it Important?
One last thing you need to be careful with is the memory Latency. This is how quickly the RAM can request and receive data. The lower the number the better. It is usually put as something like 5-5-5-12 or 4-4-4-12. You want to look at the lowest number possible but with this, costs also go up. Also, the latency goes up depending on the RAM type and speed so while DDR2 has latencies of 5-5-5-12 and 4-4-4-12, DDR3 might have latencies like 8-8-8-15. As long as you do your research beforehand, you should be alright. It isn't too much of a performance gain though so if maximising the speed doesn't take priority over the price of the product, then this shouldn't sway you too much. The latency is important for overclocking as faster timings mean that the memory can be overclocked easily but this sometimes carries hefty proce premium. If you're an average user, whatever gives you the best bang for your buck should be okay.