You're going to get the most performance boost from a Solid State Disk.
120 GB SSD
Install your OS/Programs there and keep all other files on the new data drive (your current root drive).
If you have tons of programs, then setup library/folder redirection by changing the docuement/picture/music library folders to folders on your data drive.
Humans are the only creatures that won't live up to the their potential. Give everything your all. When you die, die on E.
So I ask you, what do you intend to do now?