yeah i've already got 2 x 256gb samsung 830's in my machine now

it's just the 840's are a bit faster

I'm thinking 2 x 128gb 840 pro's in raid for boot and the 830's as a games drive and the other as a music drive.
over kill IMO
unless you're doing some heavy heavy audio processing / creation an SSD for music is a waste and RAID 0 will give you no noticeable performance increase.
Personally I'd got for 3 * 128gb 840 pro in RAID 5 as at least that way you've got redundancy.
Personally I have one for boot / applications, one for games and then a RAID 0 array for other games / scratch. But I also have a RAID1 array for my backups (which includes my music collection).