What "WiredTree Joe" told you is very important ....
Many people don't realize this but the single most important hardware component in terms of performance before anything else is memory and it's impact on your server is more significant than any other single item whether that be hard drive, processor, network, bus or anything else.
It is true that each of these other items contribute greatly to your overall performance but no where near as significantly as your memory.
When a client asks what they should upgrade when looking for better performance, the very first thing I will always look at is the memory.