lol, 60 GB for OS + apps + games..
my SteamApps folder alone is bit over 300 GB.
Better performance and more RAM for same price as your gam(y)ing kit is not good reason enough?
Also, you could compensate slowness of your WD drives by RAID-0'ing them, but, considering how "long-lasting" WD drives are, that'd be too risky.