I plan to do a pc upgrade very soon. Alongside that I plan to start with Mint or Fedora. Is there any real or big downsides to dual booting ? Aside from the harddrive space lost ?
If I like or really like my time with Mint I would probably switch permanently but I felt like I wanted to test it for a couple of months before making a complete switch.
Im a big 
so that impacts my consideration. Dont really play much multiplayer shooters so I dont have a problem with kernel anticheat games not working.
As sythas points out it is much easier to have windows and linux on separate physical drives so that you can just use the bios boot order / selection avoiding MBR issues.
Also when you are on linux try to avoid mounting or interacting with the windows NTFS partition. Windows has a tendency to throw a tantrum if it thinks someone touched it's stuff. If windows hurts itself in it's confusion then ntfsfix on linux usually sorts it out.
If you don't absolutely need windows for a specific thing though then there isn't much point dual booting. Are you really going to keep routinely rebooting your PC for a couple of months? or whenever you need to do something will you take the path of least resistance which will always be windows if that's what you're already familiar with... best to use a live usb environment for an hour to test hardware compatibility and see if you like a distro's vibe then wipe windows and dive in whole heartedly.