As for the original poster, when they are asking for 300 "dpi" most magazine just want as high of quality version as you can supply.
If the file is saved at 72ppi it doesn't really matter. How big you can print is a based on how many pixels you have, and the quality of those pixels. Lets say you have a file at 2000x3000 pixels in size (at very good quailty).
At 300ppi you could print a 6.67 x10"
At 100ppi you coung print 20x30"
Make sense?
In terms of good quality prints, I most people are all over the map in how many ppi you need to make a good print. With a VERY HIGH QUAILTY file (sharp well taken image with no diffraction or blur or iso noise etc..) a 100ppi can look good, and 125-133ppi usually looks great.
IE. Files from a M8,M9, hasselblad or other medium format etc... Xp1 files are good too.
With Nikons and Canons (even the 1D's & 5D's or D4's & D800's) the AA filter blurs the image. I find 150ppi can look good and 175ppi usually looks good. Thus why the D800E without the AA filter is so attractive.
Much of what looks good is image dependent (how much find detail there is) and also YOUR standards of what you think looks good. Personally I tend to lean towards higher quality prints. So 133ppi or so from a xp1 or leica M8/9 and 175-180 ppi from a camera with an AA filter. This is assuming its well taken as well (no bad diffraction, iso noise, lens blur, or motion blur etc...). I don't want to go to technical here, as print size, viewing distance and medium (canvas vs high quality paper), etc all start coming into effect. This is more so generalized rules if you always want a good print. These rules can be bent and broken depending on what your doing and experience. At small sizes it tends not to matter anyways, because your probably printing at 200ppi+.
I hope this helps.