Since you are replacing your hard disk and you already have partitioning in mind, it might be a good time to think about partitioning as an aid to data storage and backup ease.
This is my personal preference and is in no way the way everyone should partition their systems; but, it works for me: I have three partitions. Drive C, 15 Gigs, contains only operating system files and programs. Drive D, 5 Gigs, contains files that are subject to being added to or modified daily. Drive E, 250 Gigs, contains long-term storage of family pictures, movies, , program installations, etc. that are never modified but are added to only occasionally.
This enables me to develop a backup strategy with a little more ease. I don't have to go in and fish through folders to find the things I want backed up at various times. I know that drive D needs to be backed up more frequently than drive E. And drive C can be imaged only as my programs and configuration changes significantly.
In my case, drive D gets backed up every two hours; but, that is easy for me because I have six computers on my network and it is easy to back up one computer's drive D to other computers on the network. I back up drive E every one- to three months, depending on how much gets added.
Drive C (OS and programs) doesn't change that often so I might image it every 6- to 10 months.
Your situation may be different in that your drive C may need to be much larger; but, the principle is the same.