...200dpi will indeed be smaller as you have lesser pixel dimensions in the 200dpi images, however if your 300dpi image is in fact the same "physical" dimensions than the 200dpi image (e.g. both are 100mm x 100mm) then the only difference is how many dots get printed in the inch, they still both print the same "physical" size, just not quite the same quality...
...the best way to address this is to re-scan your images to 300dpi if you want absolute results, but you might not be able to do that or unwilling for whatever reason...
...whether you upsample the 200 images to 300 is in effect the same as placing the 200 in a 300 and physically re-sizing manually in the 300 document, re-sampling takes place which ever route you take, as photoshop has to make pixels that don't exist based upon the surrounding ones that do (bicubic algorithm is the best)...
...if these files are for high end printing then the ideal is to have them scanned correctly from the start, and scanning to a "worst case" scenario, which in most cases might be targeting 1.5 to 2 times the required output resolution...
...with scanning, more pixels is better than not enough but of course this means larger sized files to store, and with todays affordable large storage devices and quicker computers it has been much less an issue than it used to be...
...depending on the subject matter of the images you probably won't notice much difference between re-sampling 200 to 300 than if you were to scan it at 300 to begin with, but this can be subject to how physically big these images are as well from viewing distance...
...a 72dpi image doesn't always mean you can't get high res prints, if you turn off resampling and type in anywhere between 260 and 300 in the dpi field, that will tell you the physical size you can get optimum quality...
...optimum quality is pretty much determined by the linescreen an output device is capable of reproducing at, and the paper your printing on...
...the rule of thumb is 1.5 to 2 times a given linescreen in litho printing, so for magazine work typical screens are 150, 175 or 200, newspapers are around 133 or 150 linescreen, and for more challenging printing uch as fine art printing linescreens can be 250, 300 and I have known some to go as high as 400 line screen, but that is pushing it beyond what would suffice...
...for massive upsampling of low res images (for poster work) then your better off using Genuine Fractals (not free) to re-sample images up, as this plugin program is better than photoshop, however photoshop is perfectly adequate for upsampling images, you need to be careful about how far you go with it...
Andrew
