Orb, you know youre welcome to ask me whenever you got questions like these.
First the initial question:
HDDs: Its all about Buffer (2MB v. 8MB), seek time (8ms->10.5ms), size (1-250GB), RPMs (5,400-15,000) and failure rate. A seagate is gonna run you more than the average Hitachi because they make a more solid product. Its like comparing a Toyota to a BMW.
PSUs: Same can be said, but in this case its voltage, cooling, switch, speed control and (most importantly) solid feed. You want a PSU that dilivers what it claims. Some PSUs that are cheaper dont have terribly stable voltage.
As someone stated earlier, you get what you pay for, unless you buy the newest hottest thing (i.e. 15,000 RPM HDD which runs upwards of $7/GB as opposed to $1.3/GB for 7200).
Second question: You invert your PSU... that fan is meant to be the bottom, not the top. Yes, in this case the plug is upside down but thats how it goes.
System specs? Lets just get down to it people:
My penis is bigger than yours.