72dpi Printing Question

I've read some posts about this, just wanted to clarify:

I want to have a 72dpi image that is 20x30 inches printed. Should I change the dpi value to 200 with all the photoshop cs2 default settings in the image size menu? Or don't do anything at all and submit it as a 20x30 inch picture. I notice if I change it to 200dpi that the "print size" is still 20x30 and shows up at about 37% on the screen and the 100% view of the image is somewhat blurry and size of the file is much larger. Is this larger file size just utilized better by printers? Thanks!

Welcome to POTN! We have a lot of resources here that will help answer the most common questions. They're generally linked from the sticky/FAQ threads at the top of the forum. The FAQ you want is also linked from my sig, the "printing and enlargement FAQ".