How safe to get 6-8 more months out of 5-year-old MBP hard drive?

I have an early 2008 Macbook Pro that's been my personal computer for a while (separate PC laptop for work). I take it places now and then, but mostly it just sits on my desk at home, so there's little travel-based wear.

I was thinking about getting a new rMBP and could afford to do so, but there's nothing particularly slow or wrong with my current MBP. Hard drive's a little small, but 4GB of RAM works quite well for almost everything I do with it.

My only question is: How safe is it to trust that a 5-year-old laptop hard drive will keep on going for another 6-8 months or so until the next version of the rMBP comes out? I make regular Time Machine backups, but it would still be a real pain if my hard drive died unexpectedly.

I know hard drive quality is highly variable and hard to estimate, but even anecdotal evidence would be useful. (My old mid-2004 iBook is still alive and kicking, although I only turn it on every few months or so these days.)

Dark Dragoon and ohbrilliance -- thank you for the suggestions and comments. It seems like if the laptop hard drive has held out this long, it's not particularly more likely to fail suddenly due to age ("I wouldn't be more concerned than normal"), as opposed to normal random unpredictable failure.

I'll continue with the regular backups and probably also get a large-ish flash drive and start up a separate documents/photos/etc. backup on that, as suggested. Really, even though a new Mac would be cool and all, I don't particularly need one quite yet, and I actually don't want to take the time right now setting things up on a new computer and learning what's changed in Mavericks... when things are calmer, I'd enjoy it a lot more, but now it would be less fun.

(And I only think restoring from a backup would be kind of a pain because I've fortunately never had to do that before. Sounds like Time Machine makes that as painless as can reasonably be expected.)

Cool, I appreciate the advice. I'll definitely start running Carbon Copy Cloner, especially since external drives have become so cheap.