Ethan is right (for PCs)! Though with more modern operating systems, you'll need to increase the size of your system partition. Sound libraries have also increased in size.
I like a 20GB O/S partition and the rest for data. Also as RAM increases to multiple GBs, so will your page files, so you'll need space somewhere for a page file (and a hibernation file equal to your RAM if you enable hibernation) that is atleast equal to your RAM. Ideally the page file is on a separate physical disk, but as drive sizes and speeds have increased, there are usually fewer, larger drives leaving you to locate your page file on the same physical disk as the OS or your data. I'd choose the OS disk over the audio disk if you have more than one physical disk.
If I have 2 physical hard drives, I'll make the the 2nd one a single partition for audio work. This leaves me with C: (OS), D: (basic data, MP3s, sound libraries, etc.) and E: recording audio). Use an image utility to backup the C: drive so you don't have to reinstall Windows and all your programs when you replace the hard drive (make sure to make a new image after major software installations or updates). And use any number of utilities to backup the data on the other partitions/drives.
1TB drives and external USB2 enclosures are very cheap, so get one or two to use solely for backups. I like Seagate drives the most.
Large, cheap disks are great, but the failure rates are atronomically higher than on the smaller drives of 6 or 7 years ago. At the risk of sounding dramatic, I see backups as absolutely essential, not a luxury. I've had too many 200GB drives fail to not make backups.
Another side note is that system restore points in XP and Vista eat a lot of space, so I limit the space they are allowed to use, and periodically turn them off and back on to clear the space they've used if I'm desparate for space.
