When should I be concerned about Speech Delay?

I've been having my head in the sand a little, when it concerns my son, J.  He is now over 3 and his speech is still slow to come through.  He probably has a vocabulary of around 75 words and can put 2 words together (mostly Hello Mama, bye bye Poh Poh) but he can't talk in sentences.

I went to party with a group of friends who are all in the same profession, most of them are parents of children the same age as my own.  One of them, Dr C, was very concerned about J, and said I should see a speech path soon.  I was reluctant because I was hoping starting child care would get him talking more.

She sent me an article from the ABC New 24 program "Tonic".  There was this quote:

I do a little bit of a developmental screening to see whether the child seems non-verbally ok and so can the chil do puzzles and things that are appropriate at that age.  And again if the child is delayed in any of the things except talking I would say you need to do something now.  If the child is only a late talker then I would say well why don't you try to do these stimulation tips and then at two and a half let's take a look again and si fthe child is still delayed at two and a half then I might say well it seems like maybe we should think about doing something and certainly by three I would recommend speech language therapy if the child really has almost no words by three."

- Leslie Rescorla

I looked up a few other things.  From SheKnows.com, in the parenting section, there's an article called Delayed Speech in Three Year Old is a problem:

By three years of age, a child should have a vocabulary of 600 words with 80 percent intelligibility to a listener who does not know the child. This means that a person who has not previously listened to this child talk can understand 8 out of 10 words. (Parents can often understand what a child is communicating better than anyone else, in spite of any speech delay.) quotation mark open By three years of age, a child should have a vocabulary of 600 words with 80 percent intelligibility to a listener who does not know the child. quotation mark close When should a three-year-old be referred for speech evaluation? Evaluation is indicated if he or she has a vocabulary of 200 words or less, is not using short sentences, and has less than 50 percent intelligibility.

So I guess the question now is - where do I go and whom do I see?  I should be speaking 10,000 words a day to my son.  Gosh, I definitely need to improve in that regard.  And read to him more, but his attention span is so short!  I'll post updates as I progress through this trying time!