>>I don't know if setting up and using virtual machines is in your future plans, but if so you might want to leave some free space to create a partition dedicated to them. That can make it easier to back them up separately from the rest of your machine.
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>I always wanted to try using virtual machines, for testing new OS, or whatever. So you bring up a good point.
>What I don't understand is if I will need to leave free extra space on the C: partition or create a separate partition (say G:) of about 100 GB for future virtual machine.

A virtual machine is just one large multi-GB file (.VHD, .VHDX etc depending on which hypervisor you use) plus one or more configuration/meta files. They can be stored wherever you have enough free space. Most hypervisors default to saving on C: drive (assuming enough space available) but you can configure the files of any given VM to be stored elsewhere.