Since it's NPD, it's only for the US (and canada i guess).
Silent Hill 5 - 73k (360), 84k (PS3)
Silent Hill Origins: 80k (PS2), 154k (PSP)
so Origins seems to have done surprisingly well on the PSP. I thought that the PS2 version would sell a butt loads more, but doesn't seem like it .
edit : the article is dated from febuary ..
edit 2 : from the same article.
Silent Hill 2 - 512k
Silent Hill 3 - 279k
Silent Hill 4 - 209k
all PS2 numbers.