Water
Under
For others who may read this thread (because it covers a very common question) here's my take on this:
Once your machine has gotten to the age that it only meets the bare minimum specs on the latest version of OS X, you probably have a choice: upgrade some things (RAM, HD, software) and keep up one more time, or (particularly if everything is working well) stick with what you have.
You can continue to upgrade the non-OS software for as long as you eligible, and Apple issues security updates for some (just the last couple of) older system versions.
But when that moment comes, you should be aware that no matter how awesome your Mac currently is ... the clock is now ticking towards retirement. Depending on your needs it could be a couple of years, it could be another five years ... but the clock is ticking.
Most likely (and for most people), you'll want to replace the hardware sometime BEFORE it just dies utterly, so this problem sort of takes care of itself usually ... but the further behind you fall, the more chances there are that standards will have changed, software will be discontinued, and you'll have more work to do to get used to the new stuff when you finally get around to the new stuff.
My general suggestion is that if you've made the decision to stay put with the OS version you have, that's fine ... for a year or two. After that, you're probably creating more issues for yourself in the future. IOW, one probably should fall TOO far behind the current OS. As an example, I'd say people who (for whatever reason) couldn't or wouldn't make the jump to Lion as being okay for another year or so -- they'll probably continue to get Safari, iTunes and security updates from Apple, and their third-party software is likely to be supported for a while yet.
By the time 10.9 comes out however, you'll be well and truly obsolete.