The Mexican president has powers comparable to the U.S. President with one important exception. The Mexican presidency is limited to one term of 6 years. Fox is a very lame duck right now. With the exception of the 2000 election, PRI has controlled the presidency in Mexico since the 1910 revolution. However, the players in both PAN and PRI are to be found from the same 40 families who control everything in the country as an oligarchy, be it from posts in industry, as politicians, or as the heads of unions.
A PRD win of the presidency would be comparable to what Chavez has done in Venezuela. It will challenge the power structure. He'll either sell out, create chaos, or be assassinated.
You seem knowledgeable about Mexico. Can you recommend a book that describes how these 40 families came into power and how they maintain power? I know, corruption has a big part to do with it and the citizens are mostly disarmed so they cannot easily stage a revolution.