Yeah, my raptor is pretty quick too. But I was talking to my brother, and he mentioned something about this hard drive having multiple processors or something to that effect, and that combined with the age of my raptor, made it a contender for a boot hd.
I would just go with the WD Black as the Primary boot Hard drive & storage, then use the Raptor for games if you're into that sort of thing. I have the same WD Black 640 GB model and had a raptor 150 GB and could not tell the difference in booting OS, but there was much better performance in gaming.
i asked the same question as you not too long ago.i have the same hardware and wasn't sure my self and what i was told by the guys here was to use the 640 black ed and it has worked out great so far.i use my 74 gig raptor has a storage drive to hold movies that i stream around the house..good luck too ya hope that helped