Sales for the U.S. opening of Britney's Circus are currently being tallied. According to hitsdailydouble.com, as of right now (11:44am PST) with 54% tallied, Britney has sold 243,791 copies - looks like she's verrrry close to that 500,000 mark - and definitely going to be number one this week!
Will update this post when more stats are available.
UPDATE: With 69.42% tallied, Britney has sold 349,487 copies of Circus. Stay tuned for more updates.