Yeah, if I'm not mistaken, HDs with capacities of 500GB+ are much more likely to fail. That's not good, considering HDs are pretty failure-prone to begin with...
I know your pain, though. Almost a year ago now, my iMac's internal 250GB decided to just hump up and die on me for no particular reason. It contained everything I've worked on since 2001, my entire music collection, and all my anime. I was about to have a heart attack. Luckily, however, I had backed the entire HD up on my external only three or four days before that, so I ended up losing only a few WoW screenshots. Ever since then I've incredibly thankful that the impulse to back up hit me that day...