Their predictions so far have been good. Either the album sells a LOT more (P!nk, Mumford & Sons) or they sell a little less. Lets say Rih's prediction is one of the worst case scenarios and within their average of error (~35K) She can do 240K. Thats still enough to be a threat for #1.
Rihanna isn't doing that number without some major promo or marketing strategy. She'll do around 180-200k though. Tay will probably do 1 million+. Kesha is harder to know so I'll give them the benefit of the doubt with their prediction for her and One Directions prediction is nearly impossible for me to guess because this will only be their 2nd album.
My predictions
180-200k- Rihanna
1million-1.3 million- Taylor
300k-500k- One Direction
100-150k- Kesha