KnightW0lf: SSD for OS
HDD for games
personally games are just getting bigger and bigger, i have a few games that are just 50GB itself with a SDD it would fill that up pretty fast 8x50GB would be 400GB already used which also another thing you do not want to go over 90% on a SSD or HDD, i don't have the correct % though where you do want to not go over because it will both bog down both types of drives
Until you run into a huge game that is not optimized for HDD usage (though for that I could blame the developers; there is no excuse for not testing running the game from an HDD except maybe if you are a solo developer, and in that case it's unlikely that the game would get that big).
Also, one advantage to two drives is that access to one don't bottleneck the other. Playing a game off a HDD when the OS is on an SSD will get better load times than if everything were on the same HDD because the OS and game can access their drives simultaneously.
doady: This is a false dilemma. Every computer can and should have both SSD and HDD. SSD for OS and applications. HDD for temp files and documents and media.
Temp files should be on a secondary drive regardless for increased speed (both primary drive and secondary drive working together in unison). If you really care about speed, your computer should have two drives anyways.
Having a second drive makes the computer heavier, which is not good if the computer is one you expect to carry around a lot (for example, if it's a laptop). Plus, there's also power consumption considerations.
Also, on Linux, you can easily put temporary files on a tmpfs filesystem; this is faster than saving them to disk (assuming temp files don't get too big to fit in RAM) and avoids the write to the disk in the first place (good for SSDs and cases like running the OS off an SD card (Raspberry Pi)).