I have my main page which ranks very well for many terms. Google must look at it a lot, correct? Lets say I make a change to it -- and add some long tail content to it. How quickly will it rank for these terms? Does google have to come back and re-cache it?
If your main page ranks well, it would likely be fairly easy to rank well for new content pages on your site. Of course you will still want external inks to those new pages. If your main page already ranks well, I think you should use separate pages to target long tail terms instead of trying to use your main page to target too many keywords.