Don, Good NTSC video frames will be 640x480 pixels or so (i.e. 0.3 megapixels). Not sure of how your camera captures stills but if you have gotten them into Photoshop the program will tell you. When you capture images digitally, you capture pixels. The number of pixels per inch varies depending on how big you a piece of paper you spread them out on (i.e. you don't capture at ppi - despite whay you were told!). If you did nothing to your images except change the resolution to 300 dpi (under Image size type 300 into the resolution box and uncheck the "resample image" box) you would have a picture that is 640/300 inches by 480/300 inches - 2.1" by 1.6" - pretty small! If the sensor in your camera is bigger, you might have e.g. 1024 x 768 pixels, not a whole lot bigger. You should not expect high quality prints from this system unless you keep them small. You can expand the file size by interpolation - after changing the resolution to 300 dpi, you can then type in new dimensions in the dimensions boxes and check the "resample image" box and the program will create new pixels from the existing ones. Note that it doesn't create new information but it will let you print a larger print but with lower actual resolution. It sounds as though you have done something like this if your file sizes have gone up. The 20MB images you refer to come from digital cameras with several megapixels (e.g. the Canon 10D has 6MP) and therefore have 10 to 20 times more pixels than your video! Improving the quality of the image before printing it will depend on the original image quality, but you've come to the right place to learn about digital image manipulation so check out some of Michael's tips, essays and tutorials on this site - Instant Photoshop, Unsharp masking (USM) would be good places to look initially
Use the search tool to locate these or peruse the various sections and their istings
Good luck
Andy