How to determine actual competitors in Google?

How do you determine your competitors in Google? For example, if I search for green lamps I get 2,280,000 competing sites, but if I surround with quotes ("green lamps") I get 1,990 competitors.

So, if I optimize a page for the keyword phrase "green lamps" (assuming that all other optimizations factors are good and should yield a decent ranking), will I appear at the top for both variances of the search?

This is a very good question.

To me "actual competitors" are web sites that have made an effort ranking for a keyword or websites that due to it's nature are ranking for a keyword.

The only way to find competitiors are by controlling each and every keyword.

It is a bit funny really for one of our main keyword there are a non-competitor (a blog) in the fifth spot, but it is ranking high since it is a good page and many people link to it.

I totally agree with your opinion on "actual competitors", however I am try to come up with a reliable way to determine if a keyword is worth trying to market.
If "green lamps" is truly the competition then I've found a niche, otherwise I need to find another keyword/phrase that will have a greater change of yielding site visitors (buyers ), or abandon green lamps completely.

There are many types of competitors. Those with you in the organic serps... those who are with you in the ppc listings... those who are ranking for the minor keywords.. those who are in froogle and in the shopping engines... don't ignore those who are competing with you on price or shipping speed or selection as they can really do damage to your sales.

If you want to be the force in the market your goal should be to kick ALL of their asses... then tweak and be nimble where you can increase your profit. Sometimes this takes a couple of websites, each with different pricing and KW placement.

seems to me that competitors are those who rank above you in the places you care about, whether that means a certain search engine, PPC, whatever.

Don't count out the site that ranks a few places below yours. I rank a position or two below Goliath and get most of his sales on a number of nice items - I have a dynamite price in the title tag.

Also in some SERPs the searchers spend time and visit lots of sites. You can rank below someone there and still get lots of sales - if you have the best value proposition.

Finally some title tag writers don't know what they are doing.