As your asking about allocation, I'm guessing your going for a dual-boot setup on a single drive.
Changing the distro will be a fair bit easier if you create a separate partition for your user files and mount it as
/home and don't encrypt it.
Complete uninstallation should be easy enough, just use a Gparted LiveCD to delete the Linux partitions and resize the Windows one (if needed)... but you would also need to reinstall the Windows bootloader, fairly easy if you have the Windows installation disk.
GOD NO... Linux doesn't use anything as arcane as the Windows registry.
If you are installing to a drive that already has Windows installed, resizing and partitioning won't be done automatically, as you are going to have to resize you windows partition first... but this can be done from inside the Linux installer.
(I've also found it best to run a defrag from inside Windows before resizing the partition)
Allocation... most people say a minimum of 10gb, but it depends on what you are going to be using it for, I'd say if you have room give it 30gb (ish)...
10gb mounted as
/twice your RAM as
Swapand the rest (18gb ish) mounted as
/homeXFCE is a Linux desktop, not a distro... do you mean Linux Mint XFCE, Xubuntu, or similar?
be aware that although XFCE will run slightly faster than Gnome or KDE on older hardware, you may find support somewhat harder to come by as less people use it, but the choice is yours
