When did we say Na'asheh V'nishma?

One of the more confusing disagreements is found at the end of Parshas Mishpatim. Though the sequence of events at Mount Sinai seem perfectly straightforward, one minority opinion rearranges them creating all sorts of interesting ramifications.

First, here is the chronology given by the verses 

1. Exodus 19: 1 – 25
Preparation for the revelation at Mount sinai. Various rules are given.

2. Exodus 20: 1- 17
The revelation at Sinai, and the announcement of the Ten Commandments

3. Exodus 20: 18 - 23:33
The "Ko Tomar" section in which God gives Moshe a set of commandments

• 20: 18-22 Three mitzvos
• 21:1-23:33 A long set of civil laws introduced with the phrase "And these are the mishpatim [ordinances]"

4. Exodus 24: 1-2
Two puzzling verses

And He said unto Moses, “Come up unto the Lord, thou and Aaron, Nadab and Abihu, and seventy of the elders of Israel, and worship ye afar off.
And Moses alone shall come near the Lord; but they shall not come nigh, neither shall the people go up with him

5. Exodus 24: 3- 8
Moses announces the words of God and the mishpatim. A covenant ceremony is performed with blood splashed on the people. Sacrifices are offered and eaten. The leaders experience a divine revelation. The people proclaim Na'asheh V'nishma

6. Exodus 24: 9-18
Moshe goes up to Har Sinai as commanded in 24:1 - 2

According to Ramban, Ibn Ezra, Ohr Hachaim, and Rashbam this is the order of events at Har Sinai. Thus, the famous proclamation of Na'asheh V'nishma was declared after the torah was given.

There is one minority opinion that disagrees. According to Rashi, section #5 [Exodus 24: 3- 8] occurred at the same time as section #1 [Exodus 19: 1 – 25]. Thus, Naaseh V'nishma (along with the covenant ceremony, offerings and the meal and the divine revelation) occurred before the torah was given.

Why does Rashi re-arrange the sequence of events?

This is one of the big mysteries. Here are some of the answers, I've seen

• In Exodus 24:1 Moses is told to "Come up to the Lord"; however according to the plain reading, he's already on the mountain when this command is given.
• A covenant ceremony rightly belongs before the deal is executed. You sign papers, before you take possession of the house. So, too, the blood needs to be sprinkled before the Torah is received. 
• The events of Exodus 24:2-8 sound like a second revelation, which Rashi would have found untenable. 
• UPDATE: Thanks to Robert Alter, I now see that obvious and simple explanation. Its written up here

Why does it matter?

Lots of reasons. Here are some of the ramifications.

• In Exodus 24:3, we're told that Moses gave over "the words of God" and the mishpatim [ordinances]. According to the straightforward reading these are all found in the Ko Tomar section [Exodus 20: 18 - 23:33] a large part of which is introduced with the words "And these are the mishpatim" If you read like Rashi, Exodus 24:3 occurred BEFORE that section and  can't be referring to it. As a result, Rashi is forced to reinterpret the words. He says the  "the words of God" are the regulations about staying away from women, and the mountain during the days before the revelation, while the mishpatim are the Noahide laws and the rules given at Marah. He does not explain why these laws were repeated at this juncture; not having this explanation leaves his answer unsatisfactory. 
• As mentioned, it changes the timing and also the meaning of Naaseh V'nishma. According to Rashi, this declaration comes before the Torah is given and is an act of blind faith. According to everyone else, it is made after the torah was given, and means only we'll do (what we've heard so far) and we'll listen (to the rest of it).
• It also changes the timing and the meaning of the feasting and revelations. According to Rashi, these acts are part of the preparation for the revelation. According to everyone else, these acts are themselves part of the revelation. The meal is one of celebration. In fact, Rabbeinu Bachaye says the post-revelation meal is identical to the meal hosted by the high Priest upon his successful completion of the Yom Kippur service.
• Finally, it changes the view of the meal itself. According to Rashi, the meal was a sin, committed by just two of Aaron's sons, for which they were later punished. According to everyone else, the meal was a perfectly appropriate response to the wonderful events at Sinai, and it was enjoyed by all the leaders who simultaneously experienced a divine vision.