One of the problems with choosing a graphics card, is that it is hard to judge them from a few parameters. Yes you can look at the clock speed of the GPU, or how much RAM you have on board, but the performance of the card depends on so many other things, the number of shader units, texture units, memory clock, memory bandwidth. Really there is no simple way of comparing them on paper.
Choosing the best involves reading a lot of reviews for the cards in your budget range. Heck which card is the best for you might even depend on the type of games you want to play.