Put a Mac-specific RAM stick next to a PC-specific stick and they look nearly identical, so what’s the difference? The difference is that picking out proper Mac memory is determined by model while on a PC it’s determined by motherboard.
If you built your PC yourself, you shop for memory based on what the motherboard supports. If the mobo’s documentation states it will run DDR3 of either 800, 1066, 1333 or 2200, you’ll most likely go with middle-of-the-road (as most people do) 1333 memory, install it and that’s that.
You can’t buy a logic board independently for any new or currently-produced Mac computer, so the way memory is bought for those systems is by model, such as "for iMac", "for Mac Pro" and so on.
PCs are "more forgiving" when it comes to memory than Macs are. For example if you put in mixed DDR3 memory in a PC, such as one 1066 stick and one 1333 stick, as long as the motherboard supports it, both sticks will simply run at the "lower" stick speed, in this case 1066.
Disclaimer before continuing: Don’t use mixed memory in your PC. Use the same speed for all your RAM sticks, and if possible same-brand as well.
If you try to do the mixed memory thing on a Mac, OS X either won’t "like it" very much, or the Mac simply won’t boot. Some Macs are very particular almost to a fault when it comes to RAM speed; that’s the way Apple designs their computers.
In some instances you can, but more often than not the answer is no. Some will say they’ve done it and their Mac runs great, while others will say all the machine did on power-up is show a blinking power LED on the front, no picture, and no boot until they used "all-correct" memory.
The general rule of thumb is to always use Mac-specific memory for your Mac. That doesn’t mean you have to buy it from Apple specifically, but it does mean the memory must be listed as being compatible with your specific Apple system. If not, you’re just figuratively rolling the dice, you’ll probably lose, and your Mac won’t work.
Yes, and this is usually because a Mac requires fully buffered ECC (error-correcting) memory as a hardware system requirement.
The price difference between Mac and PC memory isn’t nearly as wide as one would think.
8GB of Kingston PC2 5300 fully buffered Mac memory at the time of this writing is $259.99.
8GB of like-type Crucial PC2 5300 memory for PC is $225.99.
When you go with a different type of PC memory, such as PC3 12800, that’s only 80 bucks for 8GB.
What you pay for memory directly depends on what type it is. Why does PC2 5300 cost so much on the PC side? I honestly have no idea, but you will spend over $200 to get that 8GB.
If you participate in seriously tired and old Mac vs. PC argument concerning cost of memory, the like-to-like type cost difference here was only 34 dollars. PC2 5300 costs a ton for a reason. I don’t know what that reason is especially considering the Crucial has no ECC and is unbuffered, but it busts over $200 while the PC3 12800 sits well under $100.
Maybe someone out there knows why PC2 5300 memory sticks cost so much?
Therefore, before you change or upgrade your RAM or swap your RAM consider what is best suitable for your computer and use the right hardware. For mroe information read it here