On Windows machines I've tried various approaches and come to the conclusion that it's easiest to have a single partition.
If you've got two separate spindles then, assuming you're not actually going to fill up 300Gb (ie, assuming you don't have a large collection of downloaded porno movies) then your suggestion is perfectly sensible. An off-board backup (as well and/or instead) would be better, of course.
Otherwise I'd put the OS and applications on one drive and data on the other, then backup is simply a matter of copying the data drive. Also, when you replace the machine moving a disk full of data from one machine to another is trivial whereas the OS and applications are usually best re-installed from scratch (unless you are a fully qualified BOFH and know what you're doing).