Can Toshiba NB500 (1024 MB DDR2, Atom 1,66 GHz - pseudo 64bit) cope with Ubuntu 14.04? If not, which version of Ubuntu can I use, if I want to work comfortably? I would like to use Unity if possible.
I would go with Xubuntu or Lubuntu, both will run just fine on your hardware. You could give Ubuntu a whirl, but don't expect lightning speed.
As for versions, I would definitely stick with the latest (or the latest LTS), which is 14.04.