Just buy a new hard drive, plug it in to your system, and format it (might already do it, but I forget) then tell Steam/Origin/UPlay/Blizzard to install to that new hard drive.
Also download something called SteamTool which will move all the games you want to that new hard drive clearing up space on the drive you have now.
If you want to see major performance increase I would recommend buy an SSD, they make a better increase.