ati is the better brand right now hands down...as to which brand....some brands offer voltage control and more shaders....voltage for overclocking....shaders for quality...as for price you gotta shop around...there can be a difference of 75 bucks for the same type card but different brand but bottom line the more shaders the better
I suggest Asus personally.
AMD or Nvidia is your preference. I am a die hard AMD guy, VERY happy with my cards.
It mostly depends on the game you want to play. TechPowerUp! has great charts showing all the cards' performance in any given game.
I'm currently running an Asus 6950 2g with the shader unlock. I'm very happy.