It depends are you going to:
Dual Boot to OS, or possible want the option to one day? Then 2 Partitions
Have many users documents (gigs) including movies\pics or whatever? Then another 1 for this
Now I'm quite sure most will reply with all these multiple partitions required for OS and Programs and Data and Backup (including image) and even extra one, just in case!
But personally I say just one! even on 320Gig. Partitions just allow data to be on another part of the disk, that's all. Isn't that what folders are for? Except if data is placed in folders it can be contiguous across the disk (without having these possible big gaps, that the hard drive's head has to move back and forward to)
My opinion: 1