(V): The default for NTFS partitions is that they are mounted in read-only, and with root as the owner, and you will have to change that in order to seamlessly read and write to it. I was just about to start typing out a guide, but I see that of course the Ubuntu documentation has one: https://help.ubuntu.com/community/MountingWindowsPartitions - it's actually pretty easy, just that it doesn't work "out of the box". If you have trouble with that or are unsure of details, just ask here :)

And yeah, it obv doesn't matter whether you use just a separate partition or another HD alltogether.

Artful Dodger: Yeah, what could go wrong :D I'd reserve a little more space than that to account for installed applications and app data, but 8-10 GB should be plenty to start with. Of course you can start out small - you can always load the LiveCD and load parted or the GUI version gparted (parted = partition editor) to resize later as you see fit. Congratulations and enjoy your Linux! :-)