Hello,
 
I am a low budget (so just 1 SSD)  and I have:
 
Desktop:
C: 160GB standard HDD ->OS and softwares
D: 1TB standard HDD -> Sessions
Z: 128GB SSD -> Sound libraries (I'll make it external)
 
Laptop:
C,D,E: 750GB hybrid HDD with multiple partitions
The Z drive is "substed" form a E: directory
 
I use visualsubst to create virtual drives.
 
The Vst can be on the same drive as OS since they are quite small(few Megabytes) .Most of time, they won't take a long time to load. The only thing which is slow in Vst is the loading of soundbanks.I set a Vst folder on my soundbanks drive only for vsts which load a big soundbank (vstis mostly)
 
Multiple SSDs are interesting since they use different sata ports.
An SSD for sample libraries is very interesting since big soundbanks are read direct from disc.
 
RAID is interesting,yes..A bit bothering to setup.
mirror RAID (RAID1)enable to have an automatic backup.For the sessions that's very good.
slice RAID (RAID0) enable to accelerate access to standard HDD and to use 2 disks as one. When on budget,you can use them to speed up system and sessions and get a bigger disk from 2 smallers. 
There are other RAID settings...
 
Tip:
I always put the sound banks on Z: (change this drive letter to Z) . The reason is simple: Z is mostly never used as drive letters(you'll never get 26 drives in your pc),so on any computer it will be free. That way I can easily use an external HDD to store the soundbanks, set it to Z without changing new pc config, and have my project automatically find the soundbanks when switching pc's. It's also possible to use VisualSubst to create the Z drive from a folder on any other driver.I find it to be very efficient when working both on desktop and laptop: 1 external USB3 SSD with samples,just set it to Z: and boom...you get your soundbanks loaded in you projects, on any computer, without transfering or searching.