Hard drive's probably dead, but anything else?

This is my first post on this site.

My mid-2007 MacBook was performing remarkably well for its age until about a week ago, when it died. Since I'm an unemployed grad student, I'm looking to regain use of a computer as cheaply as possible, which probably means fixing the one I have. I replaced a hard drive in this computer already, about 3 or 4 years ago. I haven't opened the computer up yet, so I don't have the specs on the drive currently in there. I had also upgraded the RAM to whatever was the max, according to the guys at the M.A.C. store in Berkeley, CA (I've since moved to Detroit, so can't go back there).

What's happening now:

I heard a little clicking, from where I know the hard drive is located, while using the computer, so I shut it off. Maybe that was a mistake, but I wanted to deal with it later. When "later" came, I couldn't turn it back on again.

Whenever I try to start it, the chime sounds, I get a grey screen, and nothing else but the clicking sound. I have (FWIW) tried resetting the PRAM; no surprise, it had no effect. If I press the power button again, it shuts off completely (no grey screen).

Because of the clicking, I'm pretty sure it's the hard drive. But since I have to beg and borrow the money to replace it, I want to make sure there's not something else I should be looking at. I can't afford to waste money with trial-and-error swapping out parts if I can avoid it.

(Although I might also replace the Magsafe DC-in board, since starting about a month ago, the would occasionally not charge unless I pressed the connector in a particular way, if that makes sense. And they're pretty cheap.)

The only other thing I've ever replaced is the battery, which I replaced less than a year ago (with an off-brand). It's still good, as far as I can tell.

I'm not very knowledgeable about computers, so please don't use overly technical language if you can avoid it! Although I can google whatever I don't understand, I suppose. :)

One more question: is it true, what a friend told me, that you shouldn't power down your Mac very often, but just let it sleep? I've also heard it's important to power it down once in a while, so how often should I shut it down (assuming I get it started again)?

Thanks for any suggestions.