Which one would you advice me to use?
If you decide on Ubuntu, here are screenshots of Gnome and KDE, to help you choose which version you like:
http://www.psychocats.net/ubuntu/kdegnomeAs you've probably noticed, we're shooting the breeze in the rest of this post about options, but personally, I would suggest to go with one or the other, then get up and running, and familiar with the programs, before you start to customize. (Crawl before you walk, walk before you run...)
If you already use Firefox, and OpenOffice, the change is easy, as they are default to Ubuntu. If you're not familiar with OpenOffice, in particular, you might think about dual booting so that you have access to MS Office, until you get used to OOo.
Though I don't really need Windows, I like to have it available. If eventually you decide that you want to go with Linux alone, you can delete the Windows partition later.
If you decide to go for it, and have any questions, just let us know. Also, check into the Ubuntu forums. Particularly the "Absolute Beginner" forum for advice.
http://ubuntuforums.org/