I go here: http://www.logicalincrements.com/ – the Logical Increments PC Buying Guide. FYI – this is one big image.
I look at the column called “GPU”…
And scroll down to the row labeled “Good”
And then at the intersection of that column and row, I buy what it says
I've been very satisfied with my results (at the time it I bought my last video card the AMD HD 6850 was in the “good” range):
Guides aren't a replacement for personal judgment and diligence – choose wisely. Still, this is a pretty useful tool. :-)