If I use the fact that 49,034 is 53% of last week's sales, I calcuated that the first week sales were approx. 92,517. Of course, the numbers are from Hits Daily Double, and they're always close but not exact to Soundscan's totals, but this means approx. 141,551 sold in two weeks...not bad for a traditional pop compilation album.
ETA: I got hold of last week's Soundscan numbers...oddly enough from a hip-hop forum. (Google Search FTW) They placed first week sales at 99,338. So if I add this week's numbers from HDD and last week's Soundscan numbers, it places total sales at 148,372. Once I find out this week's Soundscan numbers, I'll be able to give a more precise number for total sales. It could be more than what I just calculated, and it could be less, but only by about a few thousand probably.