I have some question. This would be my first time with Ubuntu,I bought some cheap notebook Asus K50C-SX002. Can I use newest Ubuntu on that notebook without any problems? Maybe my notebook is too weak for that newest release of Ubuntu? Can you help me? Peter.
Dual core processors of any type should play well with Ubuntu 11.10 and beyond. However, if your Atom is a single-core, (which I don't thinks so because you said $400), it should still be able to run Unity 2D if Unity 3D is too heavy or laggy. Try a live CD, or live USB, watch performance and if you are confortable, go Ubuntu.
If not, faster experiences may be found on Xubuntu or Lubuntu, as said by @Uri Herrera.
If you let us know the exact model of your ASUS, (or maybe somethings wrong since google gives me a Celeron laptop), we could be able to tell you how much performance could you get. Good luck! ;)