In the ever-changing computer world, one constant is that capacity and speed will go up and prices will go down. You still need to be careful not to pay too little for your components. Yes, that's right, I said don't pay TOO LITTLE.
RAM (Random Access Memory) prices drop almost on a monthly basis and there is no such thing as "too much RAM" for your computer. Because of this people are tempted to pay the absolute lowest price possible to improve the performance of their computers. This is a dangerous thing to do. In 2005 when a single stick of 1GB memory ran about $300, I would see people buying generic memory for dirt cheap. And as usual, they would bring their computers in to get repaired because now the computers are doing weird things and don't work right. I'd tell these customers that you CAN buy a gigabyte of RAM for 10 bucks, but you wouldn't want to and for good reason. Because of the extreme precision required to cram more capacity into circuits no bigger than your fingernail, there are a lot of chips that don't fully pass all the QA tests by the manufacturers. Many of those chips may pass enough tests to just be useable in very low impact applications. The chips that do fully pass the QA tests are the cream of the crop and as such they fetch a higher premium than the second rate chips that didn't pass all the tests.
The cheaper chips make their way into the market place to what I call "Bubba brand" manufacturers who will assemble sticks of RAM and charge rock bottom prices to customers for it. I recently did a search for 2GB sticks of memory and found some for $5 new. I then did a search for reputable brand names and found some for around $45. These reputable brands buy the chips that have passed all the QA tests and are much more reliable than the others.
Brands to look for when you're shopping for memory upgrades for your computer:
Crucial - they have a great tool that will scan your system and tell you exactly the kind of memory you need.
Kingston - they've been around for years and are very reliable
Corsair - They are highly regarded in the PC world for high end RAM suitable for gaming machines and servers
Going with a name brand also gives you support for the product. Most RAM manufacturers have a lifetime warranty on their products. Try getting that with generic RAM!
When you buy new memory for your Mac, you have to keep in mind that Apple only supports the RAM that they installed at the factory or at the Apple Store. This means that if you replace all your sticks of memory with 3rd party memory and you need to have your computer repaired for any reason, Apple cannot guarantee the repair. In many cases, the Mac Geniuses will remove the 3rd party memory and put it in an anti-static bag and tape it to the outside of your computer when they give it back to you. That doesn't mean the 3rd party RAM is bad or poor quality, it just means that Apple can only cover the RAM they install and sell.
While keeping all that in mind, always remember that when a factory builds millions of something, there are bound to be a few that go bad. You could pay top dollar for the most prestigious memory on the market and it still has the possibility of failing at some point. If you try to be as cheap as possible and pay $10 for 4GB sticks of RAM, the odds are you will experience more problems sooner than if you spent a few dollars extra and bought a name brand. Shop around and use some common sense. If the deal seems too good to be true, it probably is.