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.