I've done it. I put in a nice new 500GB drive in a white Intel iMac. I watched you tube videos and found sites on how to open the machine up. Luckily the machine had been opened i the past by Apple Tech's to fix the Superdrive. This is helpfull as the foil/sheilding in the inside is easy to peal back after the fact.
You also need a good range of Torx screw drivers with THIN shafts to get a few screws out.
Took me a few hours to do but I was taking my time and drinking beers
I'll be attempting the same repair myself when my new drive arrives. In addition to the YouTube videos, iFixit just opened up their repair manuals under Creative Commons licensing. I found a very thorough step-by-step guide for my iMac there.
What the iFixit guide offers that the videos don't is very clear images of exactly where each connector and torx screw is.
On the matter of doing this yourself, I agree with Lars: not a job for a novice. I've done it many times and its still a little too easy to make a mistake that could cost if you're not careful.
That said, I'd also suggest that you think instead about moving your large "media" library to an external drive. Oh, and purge your library of things you don't actually listen to/look at, which is a good habit to get into long term (otherwise, you'll just keep needing more and more storage, making for longer and longer backups ...)