Q: How can I determine the year & model for a classic Rolex watch?

I bought a used pre-sapphire Rolex Oyster Perpetual Datejust
Chronometer for about $1800 from a New York jeweler in the early
nineties and I'm just wondering what it is worth. I'm sure that depends
on its age and specific model.

Currently, the most classic of watches is lying in my "to be fixed"
drawer because my kid dropped the watch on the cement a couple of years
ago and the previous repairs ($300 for an adjustment, $800 for a
complete overhaul) are still fresh on my mind as being overly
expensive.

I'm thinking of sending this classic watch in for repair IF (and this
is the reason for the question) if I can then sell it for a reasonable
price thereafter. But what is it worth?

I did a search and found vastly divergent prices ... depending partly
on the model and year - which is why I ask this question of you
experts.

Where do I look to determine the model and year (or other pertinent
information) in a classic Rolex stainless steel & gold Oyster Perpetual
Datejust (just the number, not the day of the week) chronometer?

My only value datapoint was when I walked into a pawn shop in Texas a
few years ago (I had never even 'seen' a pawn shop until I went to
Texas - I thought they were just in the gangster movies) and handed it
to the guy just to see what he'd say it was worth; he took off the band
and looked up the number he found on the crown at that spot and said
sweetly "I'll give you $500 for this" (which is exactly when I realized
that everything in a pawn shop must be stolen because nobody would part
with their Rolex watch for that low a price unless they stole it in the
first place).

So, I can dig up that serial number.
But, then what do I do with that number?

The watch "looks" like that on these pictures I found in my searches:
http://i16.ebayimg.com/02/i/04/3c/e3/db_12_sb.JPG
http://www.swswatches.com/sws/p792a.JPG
http://www.tnsdiamonds.com/Datejust/pg1.jpg

I am pretty sure it does NOT have the sapphire crystal because the
jeweler who sold it to me said it would not fit. I also wonder if I can
get a price for a non-working watch (which is what it is at the moment)
vs a working watch (which would be a simple lookup once I can figure
out the year and model).

In summary, not being a pro, where do I find the appropriate
identification information on my Rolex watch and then where do I look
up the value for it?