This is a question that I get from a lot of friends and co-workers. Apple has got secrecy revolving around their hardware, and there is this belief from a lot of users that changing the RAM or the hard drive will void the warranty. The answer is no. You can change the RAM and change the hard drive without voiding your Apple Care warranty.
As a future MacBook Pro owner(my Mac's arriving in a couple of days, and yes, before you ask, it's been paid for in full using cold hard cash, no credit cards for us!) and as someone that's known to dabble a lot in computers, I've been asked quite a few times if changing the hard drive or RAM will void Apple's warranty. It won't. For PCs, you don't even have to ask. PCs can be customized and still be on warranty, but the belief is different when it comes to Apple laptops.
Now, even if you can change the hard drive yourself, should you do it? If you've never opened a computer or a laptop, you shouldn't. While this procedure is straightforward, if you haven't done it before, there is a danger that you'll damage something. Best let a friend or a professional do it for you, even if it is more costly.
Why the concern? Well, 8GB of RAM on a MacBook Pro costs $400 extra, and the 512GB solid state drive costs about a $1,300 more. These prices are inflated because they are available directly from Apple. I bought a base configured MacBook Pro, with the highest 2.8Ghz i7 processor. I'll customize the RAM and hard drive myself for either a 1TB hard drive or an SSD hard drive.
SSD hard drives are a lot faster, but they are quite expensive. Techrestore is offering to upgrade your MacBook Pro's hard drive to 1TB for $279, which is a pretty good deal if you're in the US.
(Image: Flickr member Tiger Pixel licensed for use under Creative Commons, Flickr member Jingyu D. licensed for use under Creative Commons)
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Just to clarify this… No your overall warranty will not be affected, but if you bring your computer in for a repair and the issue is narrowed down to be caused by the user installed hard drive, the repair depot or the store will not service the computer under warranty. Apple Authorized Service Providers may (its up to them) repair the computer.
I had a powerbook g4 and applecare (extended). When it had problems with the hard drive, I upgraded the RAM with the same parts Apple used so they didn't have issue with it. But they wanted to make sure that I hadn't replaced the hard drive with the one it came with. Otherwise the any other repairs couldn't be covered.
Unibody and the last generation of plastic macbooks (now discontinued) have a different policy then the old g3 and g4 laptops.
Apple now allows end users to change ram and hard drives without affecting the warranty. I have confirmed this with 2 different apple technicians.
Apple gives customers detailed instructions on how to replace a hard drive here.
http://manuals.info.apple.com/en/MacBook_13inch_HardDrive_DIY.pdf