If you have to ask this question, the answer is always “no”. When it’s time to “upgrade” your camera, you’ll know.
I almost don’t want to say any more, but I’m a verbose person and there’s no one here to interrupt me, so I’ll continue.
If you just want a new camera, and you have plenty of money, by all means energize the economy by spending that money on a new camera. It can’t hurt anything except to encourage advertisers to keep doing what they’re doing.
However, if you’re like most of us and have a relatively tight photography budget, you’ll want to be more circumspect. In my opinion, there should be specific reasons to upgrade/replace your current camera, and these involve questions that only you can answer.
(The same goes for buying an additional camera. Many professional photographers carry more than one camera. For one thing, they don’t have to switch lenses as often, or sometimes not at all. For another, if one camera (or memory card) fails, they can keep shooting. So, if your livelihood depends on photography, I would encourage you to get a second camera. But this entry is about upgrading or replacing a camera.)
First, can you afford to buy a second camera? I’m surprised at the number of people who don’t consider this, or who assume they’ll sell images quickly enough to pay it off. Personally, I save money in advance before making a photography-related purchase. And even then, I always consider what else I could buy with that same amount of money.
Secondly — and this one is usually a deal-killer — is something wrong with your current camera? Is it broken? I don’t mean “broken” in the sense that Ken Rockwell meant it here; I mean broken in the sense that it won’t take pictures.
Thirdly, can you name any specific ways in which your current gear is limiting your photography? To rephrase this: Is there something you need your camera to do that it can’t do?
If your “specific limitations” are things like amount of megapixels, lack of some new feature you’ve heard about, or a slightly slower burst rate, then you’re barking up the wrong tree. If you bought your camera in the past five years, it probably has enough megapixels to make very nice 8×10 prints (or even billboards for that matter — who views billboards from closer than 50 yards?)
But something like ISO limitations might qualify. Until very recently, dSLRs topped out at ISO1600, which is enough for most people. But you’re not most people. You’re trying to take handheld shots inside a dark nightclub or at a concert, or of your brand new baby doing something cute. High-powered lighting is either not allowed or would ruin the mood. Getting a camera with higher ISO options (and decent noise-control) might be advisable.
And one more question: Would your money be more wisely spent on a lens, or on lighting? With SLRs (film or digital), it’s often the lens that is limiting image quality or your ability to do specific things, not the camera. And whether you shoot indoors or outdoors, with cheap or expensive lenses, lighting is often much more important than the camera itself. Even the cheapest digital cameras can make amazing images given the right amount of light. Before blowing your savings on yet another camera that will leave you frustrated, do yourself a favor and research lenses and lighting.
If your current camera can record images, and you can’t name any specific limits, then you don’t need a new camera. If you can’t afford a new camera, then you shouldn’t get one even if you need it.