Actually I think most research shows that the number one influence in the formative years is the state of the household, and by that I mean the family dynamics and the quality of the home and neighbourhood that the child spends his/her time.
My wife is a Kindergarten teacher who has taught in one of the "roughest" cities in the country as well as "middle class suburbia". We've had many conversations about these topics over the years. I don't want to run on forever about this, but I'll drop a few relevant points I've learned over the years.
1) The comment I replied to above is pretty accurate, especially for younger children. The parents and environment the child is in are the largest influence on their academic success. If the culture of the area is not about education and success in learning, you've got a long uphill battle.
Honestly, you could pump all the money in the world into the worst high school in the country and it won't make a lick of difference if the students don't want to learn. Upping the school's finances won't change the outcomes on the level people are looking for. To get those kinds of results you need to change the culture and community around education. However, if a school/community is simply lacking resources, then it could make a huge difference.
2) Sending your child to Pre-K DOES better prepare them for Kindergarten and school in general. How much a difference one could truly make over another is not something I've ever actually asked my wife. Dinner conversation for tonight :-). I would also love to see the paper araneae had read and run that by her as well.
Education wise Pre-K will help your child with basics - colors, shapes, letters and numbers. However, the biggest thing Pre-K will do is help your child learn to socialize and interact with people their own age in a classroom setting. This doesn't sound like much, but it is actually a big deal.
3) Something not often thought about is WHEN to send your child to Kindergarten. Many schools have policies like "If your child will be 5 years old by December 31st", which may give you a choice. It is NOT always best to send your child to Kindergarten as soon as they are eligible to attend. If your child is ready, by all means send them. But if they're truly not ready, sending them is really not going to help them.
All else is secondary.