Don't sweat the 72 dpi thing. I usually reduce to around 100dpi for emailing and web upload. Some sites force you to reduce the file size to a certain size to minimize bandwith. That may necessitate that you reduce the file to 72 dpi, or reduce the dimensions of the image.

DPI and dimensions work conversly. If you scan an image at 8X10 and 300dpi, then you enlarge it to 16X20, you are effectively cutting the resolution in half. If you reduce the same file to 4x5, you will double the pixel count. There are settings in Photoshop that you need to be familiar with. One is the "resample image" box in Image Size dialogue. With the box un clicked, it will recalculate resolution as I described above. If you click the box, Photoshop will decide what to do with the pixels. If you reduce the image to 4x5, it will keep your image at 300dpi and essentially "throw away" the extra pixels. On the other hand, if you increase the size, it will continue to say 300 dpi, however, you will NOT have actually gained any resolution! It will interpolate and guess what it thinks it should put there, but effective resolution has NOT increased!

You also asked about high pass. Specifically in Photoshop, the High Pass filter is something you would apply to a duplicated layer, then blend it into the original image. An easier way to acheive that is the Unsharp Mask filter I described. It is essentially a high pass filter, that you can control the paramaters, and it automatically blends itself. Be very subtle with this. Its not intended to drastically change the image. Just enough to sharpen the edges a little.

Hope this helps.