Puppy Linux is really aimed at very low end systems (it'll work fine with about 128mb RAM!). Xubuntu and Ubuntu aren't that different any more, the desktop Xubuntu uses has become much more mainstream since it was originally designed for lightweight use. If you have 1gb RAM or more you have plenty.
Ubuntu with 2D unity or Gnome should be fine... just installed it on a old Athlon 2400 system and it ran fine [a little sluggish at times, but so was windows...]
I would personally use Linux Mint, it is the best/simplest distribution for new users to linux and is based on debian/ubuntu with changes affected by user choice. If you need any help setting anything up on Linux I would be more than pleased to help you, just PM me or something. P.S. I am a loyal Debian User