I have absolutely no explination whatsoever for the odd behavior of the 500 GB RAID, I just reformatted my system yesterday, it has done this since day one. Oddly enough it performed right where it should when it was used as single drives.
Man, those are screwy looking graphs. I had RAID0 way back in the day on a FastTrak66 card and it did something similar if I recall correctly.
Unfortuntely, the current word on the street says that RAID0 is practically useless for everyday, single user uses. Obviously this excludes video / audio editing. Both StorageReview and Anandtech recently did stories on it. Go figure, RAID0 was all the rage 1-2 years ago.
You can mount NTFS volumes under drive letters of another drive ala Linux if thats more your style. You can have a folder thats a seperate 250gig. I've never done but personally, but I know the option exists.