In my current machine, I have 4 hard drives. My primary for programs and W7 is a 300GB V. Raptor, 2 are for data, and 1 is a shared network drive for the rest in the house. In your case, it would be a great idea to have one as a primary drive (perhaps a SSD) and then a big 1-3TB drive for data or a couple if you don't want to spend a whole lot on one drive.
I have the OS on a 160GB SSD.
My frequently-changed data files are on a 500GB 7200 RPM drive
Bulk storage that rarely changes (photos, music, etc.) are on a 1TB "Green" drive.
Splitting my data files into frequent and infrequent categories keeps my backup sizes down. It doesn't really matter whether you do it via folders or drives, although using two drives lets me do some performance optimizations such as making it faster to restore live versions of virtual machines from their template files stored on another drive.
I also have several external drives which are used for onsite/offsite backup rotations and for archival files.
Thanks, think I have decided to start with 2 drives.
1. O/S drive + Major Progs - 64GB Crucial RealSSD C300, MLC-Flash, SATA3 6Gbps
2. Data drive + Minor Progs & Games - Western Digital WD1002FAEX Caviar Black 1 TB 7200 RPM
Major progs are MS Office & Adobe Suite only so hopefully 64gb will suffice.
Existing external 750gb & 250gb for backup and crap until I get a mirror
I'm at the very edge of my budget now and still need an optical drive (though they are cheap) so can't aford a third HDD, though I will probably duplicate the 1TB data drive to mirror the first at some point.
HDD #1 - 250GB - Partitioned for the OS and games/programs.
HDD #2 - 1TB - Storage
HDD #3 - 1TB - Storage
HDD #4 - 320GB - Ripping and downloading. Encoded music / video is then stored on either HDD #3 or #4.