Quality and Pricing
Apple's prices for RAM and hard drives are outrageously high. Remember, Apple loves profit, not you. Whenever you can buy your Mac with the stock RAM, and upgrade later with third party RAM from a reputable supplier. Apple buys Samsung, Hynix and Micron RAM modules in bulk, just like good third party brands do, there is nothing special about Apple branded memory..

When buying memory, it is generally not advisable to go with the absolute cheapest stuff you can find. Many Mac models have a reputation for being very picky about the quality of RAM they will work with. Low quality memory can and often will cause application crashes, total machine crashes, and data corruption. Do your homework on quality reputation up front, and be advised that while buying generic memory saves you money, you assume a greater risk of trouble later on.

You may also have issues if you use High Density RAM in your Mac (this is the most common form of RAM that can be purchased from eBay). You may find it simply doesn't work with your Mac, the computer only accepts half the amount or it runs much slower. RAM bought from a recommended supplier will not be high density and will work correctly.

It is not recommended to buy RAM faster than your machine will handle. It won't make the machine run any faster (the Mac sets the memory bus speed, not the RAM module), and there may be compatibility issues with the RAM clocking down to the Mac's bus speed. 800 MHz SODIMMs are widely reported to be incompatible with Penryn Core2Duo Macs, which require 667 MHz memory.
http://guides.macrumors.com/Buying_RAM

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Added on March 2, 2009, 8:45 pmbasically, there's nothing special about apple certified ram...

mac rumors recommend crucial, but this ram is expensive...

This post has been edited by Shin1022: Mar 2 2009, 08:45 PM

What to Buy


Example memory info in System Profiler
Before you purchase memory, you need to know what kind to get. The easiest way to figure this out is to look in the manual that came with your Mac. There will be a section on memory upgrades, advising you exactly what type and speed of memory it needs. You can also check the System Profiler utility: go to the Apple Menu, About This Mac, More Info..., then click on Memory.

Key things to know are:

Speed: sometimes expressed in MHz, but often as a "PCnnn" number where nnn can be a number like 100, 133, 2700, 3200, 4200, 5300, or many others.
The numbers are equivalent:
PC2700 is the same as DDR-333 MHz
PC3200 is the same as DDR-400 MHz
PC2-4300 is the same as DDR2-533 MHz
PC2-5300 and PC2-5400 are the same as DDR2-667 MHz
Type: here you'll see any number of acronyms, including SDRAM, DDR, DDR2, FB-DIMM, and more. You cannot substitute one type for another -- DDR2 is not backwards compatible with DDR for example. There's also ECC (error correcting RAM), which can be installed in recent Power Macs, Mac Pros, and Xserves. Really old systems may even have FPM or EDO RAM.
Format: You will usually see DIMM (desktop full sized modules), or SO-DIMM ('notebook' size modules, however all of the Mac Core/Core2Duo machines use SO-DIMMs)
Pins: different physical sizes of memory chips usually have different numbers of pins. Common numbers for current machines are 184 pins, 200 pins and 240 pins.
CAS Latency: this one isn't quite as important; the right type and speed of memory should work even if the CAS number is slower. Adding memory with faster CAS than your machine supports won't speed anything up, and adding memory with a slower CAS will probably only be a small percentage slower.
Dual Channel: many Apple machines use dual channel memory. In a few cases, this means that memory must be installed in matched pairs. Your computer's manual would say so if that's true. For the majority of cases, unpaired chips will work fine, but matched pairs will result in a slight speed increase (on the order of a few percent).
http://guides.macrumors.com/Buying_RAM