So Gimme More will be her 9th VEVO certified vid (I Wanna Go, Work Bitch, Till The World Ends, Baby One More Time, Toxic, Womanizer, Circus, Hold It Against Me) and Pretty Girls will make a tenth.
I think Pretty Girls might get Britney over 40M digital singles in the US. She had 28.665M digital singles sold in early 2012 (Janaury). She sold 2.2M singles in 2014 and I assume a similar amount during the rest of 2012 & 2013 (not counting S&S). And maybe a little more this year due to PG.
My guess:
@ end of 2011: 28.665M
2012: ~2M [X-Factor boost]
2013: ~2M [1M from Perfume + Work Bitch + BJ album tracks + OLL, 1M catalog]
2014: 2.2M [Billboard]
+ 3.5M from S&S and let's say 500K from PG and 1M catalog for 2015 =
39.865M
If PG can get to 800K or 1M that would be great.