Other equivalents are welcome as well.
I want this information to know what scanners I need to look at or at least to have the ability to determine if a scanning service will suffice my needs.
You can calculate the reolution from the megapixels like this:
Which gives you:
First, get a pixel x pixel amount for an 8MP picture: 3504 x 2336 = 8.5 mp
Second, get what size it would be in cm at 300 dpi 20x30 cm @ 300dpi
Third, take the picture to the final size: 2.4 x 3.7 cm @ 2400
You can actually do this calculation in Photoshop. Be careful, make sure they are not resampling the image to get to the size you want.
And you should thank my wife for this information :P
In terms of buying scanners, the DPI that manufacturers provide can be a bit misleading, and the actual image resolution you can get from the scanners is quite a bit lower.
The common wisdom is that most consumer flatbed scanners top out at an effective resolution somewhere around 2000-2400dpi, despite reporting numbers as high as "4800x9600" (like my Epson 4990). Dedicated negative scanners come much closer to their reported dpi, though still generally don't quite make it.
But even that isn't quite the whole story: scanning at the higher settings (4800) may not get you an improvement in pure resolution, but if you reduce the image to something closer to the 'true' resolution (2400), you effectively get a super-sampled scan, and can see improvements in noise.
Maybe to sum up: a flatbed scanner will get you a quality print from a 35mm negative up to about 8x10, and a "pretty good" print might stretch up to about 16x20. And don't forget the influence of the printer...