Deepwater Valdez

400 million gallons = 40 Exxon Valdez

Exxon Valdez spilled an estimated minimum 10.8 million US gallons (40.9 million litres, or 250,000 barrels) of crude oil.

Why BP Won't Measure the Oil Spill

Independent scientists analyzing the slick set the estimate at 25,000 barrels a day, and once BP released the underwater video, they calculated flow rates as high as 80,000 barrels a day.

From a comment in that thread :

at 80k barrels a day, this very well could become the most massive spill in history - EASILY. That's 3,360,000 gallons a day for 90-120 days (assuming the other wells they're drilling do what they think they will - which is another assumption they're making). That's 403,200,000 gallons, dwarfing the Persian gulf war spills in the early 1990s.

A couple of things with this estimate approaching almost half a Billion gallons of oil.