Years ago, I read that film is about 6,000 dpi. i.e. 6,000 bits of silver per inch that could be turned black when hit by light.
A 35mm negative is 24mmx36mm, so call it 1 x 1.5 inches. Or 51.5 MP.
I get why you ask, believe me, but as the other posts in this thread show, there's much more to the story than that. Coming from a film background, the question becomes: At what point is digital sufficiently good enough to equate to film, such that it's time to buy in. My best guess is that we hit that point somewhere around the 6 - 8 MP range.
Easy answer - go buy a 6-8 MP camera as cheap as you can, and go play. Yes, it's not the same as a DSLR camera / capability wise, but it will let you explore and answer the question, practically. When you buy your real camera, whatever / whenever that is, hand this one off to a grandchild, niece, nephew, Little Sister/Brother, whatever. It's disposable. In the meantime you will have had a chance to figure out what the new process that occurs is, after you press the shutter button.
The problem with film (now) is, the processing and distribution of the results is now all by computer. No more chemicals and darkroom. No more printing of 4x6's (and processing costs and time) - you show people your pictures on the computer, via e-mail, or via posting to flickr (or something).
All bets are off for pros, be it magazine or billboard, or studio shoots, but that is likely not you. (Odds are.) Even there, where 54MP medium-format backs are probably used, the results will be sent and processed electronically, and sent to a printing device that gets its input via computer. Still, no chemicals.
Even movies are headed more towards HD video than 35mm film. (I'm guessing.)
To say that 51.5 MP is the digital equivalent to film is ridiculous. In a sense. As this thread shows, there's so much more to the equation than that. But until you try digital (say, 6-8 MP) you won't be able to diminish the MP question far enough to answer for yourself 'How good is good enough?' and get on to other equally or more important factors. Such as 'Interchangeable lenses?'
Printing, say, a 16x20, would be 22 MP (266dpi, the human eye can't see detail finer than that, from any reasonable distance), shows that a 51.5 MP file buys you nothing. Combine with advances in the processing, within the camera, within the computer application, and within the printer, says that something somewhat less than 22 MP will produce something indistiguishably 'good enough'.
But, you're not likely to print too much that big, film processing is dead enough to be an inhibitor to taking any pictures at all, so just go buy a P&S for those family pics, and get on with your day.
The only real decision factor, to my mind, is - how far do you typically blow up pictures? e.g. If you take a family shot from a fair distance, and want a head shot of a particular individual, will you still have 300 dpi of resolution in your original image to produce the print size you want?
I think, somewhere above 5 MP, will have enough.
I don't think one can answer for themselves "How good is good enough?", without buying something, anything, and experiencing it, to reduce the significance of that question by the context of the many other questions that will arise. Just go buy something, anything, knowing that it's disposable and you'll be buying something else within a reasonably short period of time. Besides, it's always good to have a backup camera around. Even if only to hand to a buddy to say take some shots from a perspective different than I am.
