If you play Doom3, then NV cards generally have a three or four position advantage. Same for pretty much anything else OpenGL.
If you play Half Life 2, then ATI cards generally have a thre or four position advantage.

Stupid
These cards tend to perform very very close to each other. Which is moronically fast.
Radeon X850XT PE
Radeon X800XT PE (Very slightly faster memory)
Geforce6800 Ultra Extreme


Exceptional
These cards will play any game of today or tomorrow with detail settings very high or maximum and at high resolutions.
These cards will not be cheap and neither will the Pentium4EE or Athlon64FX required to keep them fed.
Radeon X850 XT
Radeon X800 XT
Geforce 6800 Ultra
Radeon X800XL (Unavailable)
GeForce 6800GT

Great
You won't be complaining with one of these cards. In fact, you're unlikely to have a CPU capable of making them the weak spot.
Radeon X800 Pro
Geforce 6800
Radeon x700 XT
GeForce 6600GT
Radeon X800SE
Radeon x700 Pro
Radeon 9800XT
Radeon 9800 Pro (9800+)
GF FX 5950 (5900U+)
Radeon 9800
GF FX 5900U


Good
You won't be wanting an upgrade for a while with one of the following cards, but you probably won't be winning any pissing contests either. This is roughly "mainstream" for our purposes. If buying new and to stick with the card for a while, this is the category you should be looking in.
Radeon 9700 Pro - 9700s core clocks tend to overclock very very well, but
Radeon 9700 - RAM tends not to overclock much.
GF FX 5900
GF FX 5900XT/SE (5900-)
Radeon 9800 SE
GF FX 5800 Ultra (5800+)
GF FX 5800 (4x2, 128 bit)
Radeon 9500Pro (8x1, 128bit)

Average
These cards won't win any awards, and won't really attract many buyers, but they are to be taken seriously. They'll run most games at respectable frame rates. If you can pick one up used, you won't regret it.
Radeon x700
GeForce 6600 (Beats the x700 in Doom3)
Radeon 9600XT (Pro+)
GF FX5700U (5700+)


Below Average
These are passable cards, but nobody is going to want one for gaming on.
Radeon 9800SE
Radeon 9600 Pro (9600+)
GF FX 5700
GF4 Ti4800/4600 (4400+)
GeForce 6200


Poor
These cards have simply been outclassed by too many cards to make them worth considering but they do have strengths and are servicable.

GF FX 5600U (Only consider the new version, which is much improved)
GF4 Ti4400/4800SE
Radeon 9500 (4x1, 128bit)
Radeon 9600 (4x1, 128 bit)
GF4 Ti4200/4200SE (4400-)

Awful
Not really worth considering unless it's for a very good price and you're not wanting to push the envelope anywhere. These cards should be considered as bare minimum monitor drivers and old-game players. You will be reducing settings for newer games. Note that the listed FX series cards here all do *much* better at DX7/8 games than they do at DX9 games, but are still inferior to the GF4 Ti series.
Do not buy new.

Radeon 8500
GF3 Ti500 (GF3+)
GF FX5600
Radeon 8500LE/9100 (8500-)
Radeon 9000 Pro (9000+)
GF3
GeForce4MX 460 (DX7 only)
Radeon 9000/9200
Radeon 8500 OEM ("LELE", 8500--)
GF3 Ti200 (GF3-)
GeForce4MX 440 (DX7 only)
GF FX5200U

Laughable
Now these are even worse than crap. They're an insult to the market.
GF FX5600XT/SE
Radeon 9600SE
Radeon 9200SE
GF FX5200 Be very careful with FX5200 cards. I have seen SDRAM based cards which are so painfully slow that an old GF2MX would make them look silly.
GeForce4MX 420 - I had a Voodoo3 which ran faster than this.