Also, I must say the customer support from ifixit is great. I messaged them about this and some other issues I was having and I got a detailed response from them within a couple of days. Thanks guys!
I had no trouble installing the 4 GB RAM I bought for my Mid 2009 15 Inch MacBook Pro. The mistake I made though was not realising that my machine has two RAM slots. I only bought one chip! So now I have to buy another one.
If you read this before you buy some RAM, make sure you check how many slots your computer has!
I have just gotten my second hard drive up and running after a few false starts. The main piece of advice I'd give you if you are going to do the same is, format the new hard drive via USB BEFORE you install it!
Physically installing the second hard drive was no trouble at all. The problem I had was formatting it inside the computer. This gave me multiple failed attempts using Disk Utility. When I tried to erase or partition the drive, after a long hang, I always got the error: "File system formatter failed."
I thought something might be wrong with the hard drive, and was about to return it. But then I thought I'd take it out of the computer and plug it in using the Silicone 2.5" Hard Drive Enclosure with USB 2.0 Cable. I ran erase in Disk Utility again and it worked! Now it's showing up in finder and I can start storing files on it.