Answering directly to the title of the thread, just a subject GRE score is not going to open the doors to top universities. It is my impression that it is used as a filter and is not by any means a substantial factor in their decision. By that I mean that GRE scores do not matter, as long as they are "high enough" (i.e. somewhere in the 80s range would be my guess for top programs).
So it all largely depends on the other aspects of your application (letters of recommendation, grades in advanced classes, overall academic standing and aptitude for research / graduate study). That being said, I also think that people with very high scores (say 95+) are also more likely to have a strong application overall.