Personaly i wouldnt recommend buy a ati card. Ati drivers sucks and some games dosent work well on ati card. It is better to get a GTX 260 or GTX 280. And no you dont need a high-end and very expensive pc to play all games. My pc for example( that as you can see it is not so powerful) can play all games on max settings( all EXCPECT Crysis)
But i am not going to change it just for crysis, I will buy a new one in year.