Spreading Keywords to Thin

We currently have a website with 500+ different items. Being that there are so many different items, it would seem that I am trying to spread a lot of keyword content on one big site. I wonder if this might make our keyword and text content seem to thin. Thus not keeping us from getting very good rankings on some different keywords.

To give you a run down, we have about 10-20 keywords where we are sitting either in the number one spot or in the top ten. I am trying to get some of the other keywords that arent doing so well at higher SERPs.

Also, there is the possibility that some of our potential customers are searching for the singular keywords that we might be missing. What could be done to help to pick up these customers too?

Thanks in advance.

Top Level pages tend to be for very generic versions of what you offer e.g. cars

Item pages should be very specific as these are your "money-makers" e.g. Ford tauris with 4 wheel drive

Repeating the same thing isn't a good approach since their are literally 10s of thousand of keywords you could optimize for and all could have a buyer at the other end.

Don't get stuck on "I need the most competitive term - because that has the most use" -- you will be disappointed with the results e.g. 10, 000 visitors per day, zero sales (what's wrong?) those were not your target market.