Keeping the OS, programs and plugins on the main hard drive and making that an SSD allows those data intensive aspects to load and run faster. I chose this method from my own research and advice, and my experience has proven it to be true.
Projects on a separate drive increase data throughput as there is now another drive (with read heads) to load up data.
Samples are often streamed from disc and even though it would be nice to have them all on SSD, it seems a little costly at the moment for the terabyte size that sample collections can soon reach. SATA 3 is fast enough to stream samples, go to 10krpm disc speed if you have concerns.
OS disc 250 to 500 gig. I have 250 SSD on my Studio PC while I have 500 HDD 3 on my laptop where I have all sorts of programs imaginable from Office to Adobe suites and assorted other Daw, Video editors and plugins for video and audio work with only about 65% space used.
Project disc 250 gig. SSD or HDD Additional projects can easily be archived to other discs.
Samples 2TB or larger minimum 7200rpm rotational speed.
Something common to all disc is that they should run on SATA 3 ports that is 6GB. HDD should be 7200 rpm.
A limitation you may come across is how many SATA 3 ports your motherboard has. Mine only has 4. Of course if your motherboard doesn't have SATA3 ports you'll have to be content with SATA 2.
Have a good back up strategy, some folks use RAID set ups but I haven't seen anything to convince me that it is of particular benefit to audio.