I think the simplest way to determine what card to buy is decide your budget beforehand. THen, go look at some benchmarks to compare the different vid cards, and how they perform. Dont really bother with counting pipelines, or MHz/GHz.
Then, find the best performing card, whether it be ATi or Nvidia, for what you have set your budget.
I have found that the VGA Charts
from Tom's Hardware were pretty good at lining up all the cards for a good comparison.