I'm still a little new to all this stuff, But this is my understanding. Please correct me if I'm Wrong.
You usually want to buy the newest video card because it will have the newest technology on it, Like smartshader 2.0 (old) to smartshader 3.0 (new)
"What are the biggest factors" Well....You want Pcie because its 16x, New games now require more ram, So get 256+.
ATI and Nvidia have taken different approaches with pipelines and what not, Ati is using less pipes with more speed and Nvidia is using many pipes with less speed. So, you can't look at just pipes or just speeds, You need to read reviews, look at bench marks, and ask Questions here
It definitely matters what brand you buy because the cheaper brands use lower quality parts to make there video cards. You get what you pay for
I wouldn't say the cheaper brands are nesasery bad, But the more expensive brands will probably be a little better and last a little longer.