I have a 2.8 Ghz 2008 Mac Pro that currently has two 1 TB Seagates, one being my main drive, the other being my Time Machine drive. I've run out of room, so I need capacity, but I also need my new storage to be reasonably fast because of my usage.
OK.
😎 The '08's are easier to deal with when it comes to RAID, especially if it's via a 3rd party card using the HDD bays.
I'm a classical composer and I bought my Mac Pro to run the Vienna Symphonic Library, a 270 GB (and growing) behemoth sample library, which I use in Logic to realize the instrumental parts of my compositions in progress, or as a final rendition for those pieces which don't get performed live. As such, I need a lot of storage (as the VSL isn't my only sample set), but I need to be able to stream the samples rapidly to avoid stuttering.
Ideally, it sounds as if you could really benefit from SSD for an OS/application drive (or even an array), and use RAID for storage. But given your capacity needs for the libraries and limited budget, it's not yet feasible.
So the least expensive route is to go with a stripe set (RAID 0), and make absolutely certain you've a backup system in place (schedule backups in the software, as it's easy to forget to do them manually).
I still don't know what your budget or throughput requirements are, but given a single SSD is capable of ~220 - 250MB/s reads, I'll use that as a target.
So a 3x drive stripe set would be able to do this (sustained, not random access, which is what Logic would really benefit from, but it's still better than a single drive). That leaves you a single HDD bay for a backup drive, say a 2TB WD Green Power.
Hopefully, we'll get you dialed in as best as possible for your budget. Just keep in mind, random access though improves with a stripe set, is still no where near what an SSD can do.
You might be able to get the OS and Logic on a single SSD, and use a stripe set to hold both working data and the libraries. You need to think about this, and let me know if it's a possiblility.
Remember, a stripe set can produce n*single drive throughput = array throughput (i.e. 100MB/s for a single mechanical drive * 3x disks = 300MB/s).