As far as I know, you get:
100 pts=2 green squares
200 pts=3
300 pts=4
400 pts=5
500 pts=6
600 pts=7
After that, I don't know when an 8th green square appears. Just from personal experience, (again, if I may say the forum, Political Hotwire), I have over 800 pts there and have 7 green squares. (After 700, then 800, the number of squares didn't change).
Also, if you have noticed, if you put your cursor over your squares, there will be a saying about you. Here is a table of how many points you need for the sayings to change:

In the above example, we have all of the levels that users on this forum can achieve. For example, a user with a reputation of
55 would have the level of
will become famous soon enough. A user with a reputation of
-5 would have a level of
has a little shameless behaviour in the past. Since the lowest level in this example is
-99999, any user with a reputation lower than this would use the default undefined level that is set in the reputation section of the
vBulletin Options.