Your first SSD should always be your boot drive. That'll make a big difference in how your system "feels."
Here's how I've done my rig:
- 250 GB SSD for boot drive and general programs.
- 500 GB SSD for Kontakt and EWQLSO libraries.
- 2 TB RAID0 array for loops, samples, stems, and Steam games.
- 500 GB HD for my cloud programs (e.g. Dropbox, Google Drive, etc.)
My laptop runs with a 256GB SSD and I have most of the samples and libraries on a 1 TB external 7200RPM USB drive. It's definitely slower to load up projects than my desktop, but it's livable, so if you're short on money, just go for an SSD boot and see what you have left for storage stuff. I do recommend going RAID0 as a low-budget option if you can, since you get effectively double the speed of those hard drives.
Also, BACK UP EVERYTHING RELIGIOUSLY! At this point in time, I have at least three separate backups of everything that's important, so if a drive fails (and if it's a drive in the RAID array, it'll take down the data on the other one as well), I'm not screwed.