Well, here is how I see it.
The dojo, believe it or not, gets as much traffic as Walmart.com. That's right. Although it's not a fair comparision, the Halo 3 site at it's peak (the day of Halo 3 release) had ALMOST half of what Brawl has on average. Dojo has updates though and that is why it is constant, but the fact is, on average it has the traffic to compete with a massive retailer like Walmart.
Brawl has hype, there is not a doubt at all there.
Will it beat Halo 3?
Preorders: Doubtful.
Total Sales: Definitely possible. Wii is selling like hotcakes even after a year, and is the fastest selling game system of all time. Many people who buy it will get Brawl, because it's not a hardcore game like Metroid. Kids will play it at their friends house and get their parents to buy it.
Halo 2, which would be compared to Melee, has only sold about 1-2 million more copies than Melee, and Melee was on a console that was considered the least popular of the generation. Now you have the most popular console, with an epic game like Brawl, it certainly has a chance.