This image is about 6" wide. It was shot on 120 medium format film. That's a magnification factor of 2.7 give or take. It's fantastically detailed and reasonably sharp. It looks even better on paper because on paper it will be 3 to 6 times the resolution that you can see on your monitor at 100 dpi or so. So how many megapixels is that? It really doesn't matter as long as you can get about 250 - 300 dpi on paper. Your magnification may be greater because you probably have a sensor that is DX or FX sized (Nikon speak for APS-C and Full frame 35mm) so your magnification would be like a factor of 6 shooting full frame 35mm.
So what's all that mean. Well it means if most of the stuff you ever print or display is about 6" up and down you are great with about 2.5 megapixels - seriously. Oh but I want to display it on my HD TV okay - you need about hmm, 2.5 megapixels for that you can do that math to show you how big a really good print you can get is. What do you get with more megapixels? You get the ability to make bigger prints, that's it. At the same time you do get to look at how lousy your technique is, how bad your lens is, and clog up your harddrive faster. What happens when you look at a 12 megapixel image at 100% in your favorite software assuming it was taken on a full frame or 35mm device? Well you get to simulate what a 30"x45" print looks like at about a foot away - more importantly you are looking at your image at a factor of 30 magnification. Let's go for the gold and do the same thing shot on a 25 megapixel D3x for $8000. Now we are looking at a print about 40"x60" print at 40x magnification.
Let's see how my crappy old film image holds up looking at a 40" print. Wow! amazing - that is like a giant print I will lay down money that it fares better than the D3x at the same print sizes. Why you ask? The answer is that magnification factor that I keep mentioning. You see 25 megapixels at 100% on an FX sensor is 40x, about the same as the microscope shots that they used to put in all the photo magazines to test lenses and films and such. That magnification factor is pretty important when revealing lens flaws, technique flaws, etc. My crappy old medium format negative at 40x would be like 8 feet wide a whole lot bigger before those same technique and lens flaws started showing up.
Here is my money saving tip for those with gear lust - before you upgrade megapixels (assuming you have something that is already enough) ask yourself what the purpose is? Are you going to make giant prints? If so by all means upgrade but also realize that you really really have to upgrade your technique by a large factor when you get more pixels - maybe your lenses to. If you are not really focusing very critically forget lenses. If you are handholding your shots at anything but stratospheric shutter speeds, forget lenses. I am telling you from decades of practical experience here that to up the quality of your game it takes a lot of effort and time on your part. If you have any hope of seeing bigger and better prints in terms of quality you really need to completely change the things that have to do with the way you focus, expose, and shoot prior to upping your magnification (megapixels). You may actually be far better off with less magnification.
If you do want to make the occasional really fantastic large print look into film, even medium format film. Use it when you really want to concentrate on making photographs. The kind of kit that you can pickup now in 6x6 or 6x4.5 for chump change is absolutely mind numbing. Hasselblad/Zeiss for $1500 that looks like it was never used with the latest (or next to it) CF/CFi lenses. Bronica ETRsi for those that like to hand hold full kits with a couple of backs and a lens or two - $500. I have even seen some Hasselblad H1 film kits with backs, prism, body, and a lens go for $2000. I promise you that this stuff will blow the crap out of a 25 megapixel full frame 35mm body. Don't like the grain/noise - well that's fine considering you can only see it in a 40" print like shown here and that is 400 speed black and white old tech film. If you are looking to up the ante in your print size/quality it is a far less costly and more productive route - and actually a little easier than going with more megapixels at higher magnifications. You will have to shoot a heck of a lot of film to make up the cost.
The two brands that I mentioned I can vouch for personally in terms of consistent quality, esp the Hasselblad and Zeiss CFi stuff. Just something to think about. The really funny thing is that this particular negative is just a casual working shot that I happen to be printing out for a book concept that I am working on, not like a demo quality negative. It was shot at 1/8 second without mirror lockup on a 503/CW - translation = bad news if you really want critical sharpness and detail.