If the images are correctly proportioned you just need to go to Image... Size and uncheck Resample Image. Then change the Resolution to 300 pixels/inch and click OK.
Nothing will seem to change, but the DPI info inside the image will change.
Now click on File... Save As... and choose JPG. Save it at a Quality of 12 for maximum quality.
If the images need cropping, too, then this sounds like a great use for the Crop tool...
Select the Crop tool from the toolbar, then up top you'll get some options:
(This is from Photoshop CS - I think it's the same in PS 7 though.)
For Width and Height enter the size you want, whether 4x6 as shown or 8x10 or whatever. Make sure that you type "4in" and "6in" to make sure you get Inches. For the Resolution, enter 300 for 300 DPI.
Now draw the crop how you want it. The settings you choose will keep the correct aspect ratio. When you've selected the image how you want click on Image... Crop. PS will crop it, and then resize it to 300 DPI.
Now File... Save As... and choose JPG, selecting a Quality of 12 for maximum quality.