A:
You should have no problem upgrading your Dell with Windows 8.1 from Windows 7. Everything you need is included on the install discs or online when you install. Be sure to backup your bookmarks and other settings.
2 gigs of RAM is fine, though if you can afford some new RAM and you are comfortable installing it, I would get two sticks of 4 gigs and boost your overall preformance.
51 gigs should be fine (my windows folder only takes up 17 gigs)