SEO: search-friendly URLs worth it? possible without big redesign?

I’m working on an extensive site containing about 1000 PHP files. In addition to search pages and account pages and so on, a great many of these files accept numeric query string parameters:

[code]http://example.com?id=1234[/code]

Which might load up a school named “Occidental College” or perhaps a Career such as “Computer Programmer – Applications”.

We are considering trying to alter the structure of our site to present more user-friendly URLs like this instead:

[code]http://example.com/careers/Computer_Programmer_-_Applications[/code]

.

I completely understand how we might use mod_rewrite and some database work to map that URL onto [url]http://example.com?id=1234[/url]. However, I’m wondering a couple of things and would like some input.

1) If we don’t change our internal links to a page (i.e., they still point to [url]http://example.com?id=1234[/url]) then this doesn’t really help our search engine ranking unless external sites somehow know to link to the new fancy URLs, right? The perhaps futile hope is that we don’t have to go change all the links in our 1000 PHP files and that we might use some mod_rewrite trickery to gain some advantage here.

2) Suppose I type [url]http://example.com?id=1234[/url] into my browser. Is it possible to display the search-friendly URL ([url]http://example.com/careers/Computer_Programmer_–_Applications[/url]) in the browser instead so that if someone copies it to an email they get the long fancy URL?

2a) Is it possible to efficiently redirect a search engine looking for [url]http://example.com?id=1234[/url] to [url]http://example.com/careers/Computer_Programmer_–_Applications[/url] and thereby gain all the advantages of our supposedly search-engine-friendly URL? Would we need to send an HTTP response code (301? 303?). Will the search engine be smart enough to know that the search-friendly URL is the ‘real’ link when it encounters the old-style link in the HTML of our site or somebody else’s site? Does it help our ranking at all? What is the best technique for this?

3) I’ve heard that Amazon went from the query string / id approach to search-friendly URLs by using mod_rewrite type technology and ignoring the middle search-friendly part of a URL and putting the id part at the very end of the URL like this book called “Choke” by Chuck-Palahniuk.
[url]http://www.amazon.com/Choke-Chuck-Palahniuk/dp/0307388921/ref=sr_1_1?ie=UTF8&s=books&qid=1238107513&sr=1-1[/url]

This is what I think I should be shooting for and I understand how to accept the URL on our server and load up the right content, I do not understand how to capitalize on it to improve our search ranking. Thoughts?

4) Will this really help that much?

Any and all discussion of this is welcome and encouraged.

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