RAM for gaming
RAM makes a difference, there is no denying that. Fast RAM and larger amounts of memory will increase the speed of your computer. Especially when large amounts of data are being moved about, more RAM means you have to access the "swap file" on your hard drive. RAM is much faster than accessing your hard drive in most cases, so accessing the swap file is something you want to minimize.
That said, on a gaming rig, RAM isn't going to be the be-all end-all importance compared to some of the other components.
There is certainly a minimum amount of RAM you would want on a gaming rig, and this amount is going to change based on current games, but 4 to 8 gb are going to be enough in most cases.
This is not a "break the bank" item that is going to cost an arm and a leg. RAM is relatively inexpensive nowadays. Thirty dollars can get you two 2 gb sticks or 4 gb stick, and 60 will get you four 2 gb sticks or two 4 gb sticks.
At some point, more RAM is not going to make a noticeable difference. I've stopped at about 8 gb on my gaming machine, and I haven't had a problem yet. If you run more intense gaming, you may need more.
What kind of RAM to buy?
The kind of RAM you buy will be directly related to what motherboard you purchase(to be covered in another lens, look for it in the future).
You must buy RAM that matches the slot type in your motherboard.
Most motherboards today are running DDR3 RAM. DDR3 RAM stands for Double Data Rate(version 3) Random Access Memory. Sounds confusing, but actually is quite simple.
It's very similar to a car generation. You may know the difference between a second generation Corvette versus a third generation Corvette. It is the same principle, comparing DDR2 to DDR3.
Picture Credit: Martini
Modules have two ways they normally express speed. Sometimes you will see them labeled "DDR3-800." You may also see them labeled PC3-6400. This does not mean the 6400 is 8 times as fast as the 800. These are different ways to express the same value, similar to how 10 millimeters is the same as 1 cm.
Think of a car going down the highway. How fast is the car going? 50 mph? 60? 70?
DDR3-800 would be driving about 50 mph, DDR3-1066 would be going 60, 1333 would be going 70, and so forth.
If you follow this link, it contains all the different DDR3 types and list all the speeds.
Most of the time, your motherboard will figure out how fast to run your RAM correctly, and without any intervention on your part. The motherboard should slow the speed of your RAM to the slowest stick, if they are mismatched.
If you are building your computer now, it is a good idea to buy identical sticks, or possibly a "kit" of 2 or 4 sticks that match. I will explain why next.
Why to buy paired memory sticks
Generally, your motherboard will have 2 or 4 memory slots(occasionally 3, 6, 8). If you are building your computer now, it would be a good idea to buy memory sticks that are paired to increase the speed that your computer can access data.
Newer mother boards have memory slots enabled that allow you to have "dual channel" memory, if you have paired(identical) memory sticks.
Dual channel memory allows your memory sticks to be accessed together, speeding up memory access. Think of a 2 lane road getting widened to a 4 lane road.
As you are building your system, it is important to plan ahead. Do you have 2 memory slots or 4 on your motherboard? The picture to the right shows 1 memory stick in 1 slot with 3 open slots.
If you are the kind of person who is "always upgrading" your computer, consider buying 1 larger memory stick that will only take up 1 slot. Later on, you can upgrade with an identical memory module that will complete your dual channel.
If you are the kind of person who will want to put it together any be done, consider buying multiple, smaller memory modules, in multiples of 2, generally. Smaller modules are less expensive than larger modules, but you will have to buy more of them.
Triple channel motherboard/modules exist as well. If you get a motherboard with triple channel, buy in multiples of 3. These motherboards will have 3 or 6 slots.
Watch for deals
Look for deals! Sometimes you can find a faster memory stick that costs the same as, or even less than a slower one. Sometimes you can find 4 gb stick for the price of a 2 gb stick.
You can often find kits with bundled games, or even a "buy one-get out" deal, and make out like a bandit.
If you have the patience, biding your time will pay off and you can score a great deal!