`gparted` is a nice GUI tool for resizing partitions, or ext partitions at any rate. I have not tried it on NTFS filesystems, although apparently it can.
So yes, you can resize now or later. Just backup your personal tish first, just in case. Of course, if you know what is good for you, you keep that backed-up anyway ;)
Note that you should not resize a mounted partition, so if you want to resize `/`, you will have to boot a live CD (I'm sure they all have gparted) and do it from there.