Updated 20060523
To clear up confusion, here are recommended video cards for Nexuiz:
NVIDIA GeForce 7900 series - awesome
NVIDIA GeForce 7800 series - awesome
NVIDIA GeForce 7600 series - awesome
NVIDIA GeForce 6800 series - awesome
NVIDIA GeForce 6600 series - good
NVIDIA GeForce 5950 series - decent
NVIDIA GeForce 5900 series - decent
NVIDIA GeForce 5800 series - decent
NVIDIA Quadro FX 1400 series - decent
ATI Radeon X1800 series - awesome
ATI Radeon X1600 series - good
ATI Radeon X850 series - good
ATI Radeon X800 series - good
ATI Radeon X700 series - decent
ATI Radeon X600 series - decent
ATI Radeon 9800 series - decent
ATI Radeon 9700 series - decent
NVIDIA GeForce Ti4200-4800 series - ok (lacks OpenGL 2.0 shaders)
ATI Radeon 8500 series - ok (lacks OpenGL 2.0 shaders)
Any other cards are probably not well suited to Nexuiz and should not be purchased with Nexuiz in mind.
Warnings about some cards that are especially bad: (DO NOT BUY THESE CARDS)
NVIDIA GeForce FX 5200 - slow
NVIDIA Quadro FX 540 - slow
NVIDIA GeForce 6200 - slow
NVIDIA GeForce 6600 LE - slow
ATI Radeon X300 series - slow (especially X300 SE)
ATI Radeon 9600 SE - slow
ATI Radeon 9100/9200 - slow
Mac Mini - extraordinarily slow
Intel Integrated Graphics Processor (IGP, mostly found in notebooks) - very slow (You will usually need to turn off particles)
Onboard desktop graphics in general - slow
NVIDIA GeForce 4 MX series - slow
NVIDIA GeForce 2 MX series - slow
ATI Radeon 7000 series - slow
Matrox Parhelia - assumed to be slow (untested)
Tips on making Nexuiz playable on slow cards:
1. First turn off all lighting effects in the menu, this is often enough to make the game playable on any card (including the ones listed above as 'slow').
2. For even slower cards, turn down particle quality (I do not advise turning off particles entirely as doing so can make the game more confusing to play).
3. As a last resort (on for example a Mac Mini), use r_showsurfaces 1 (available in Nexuiz 1.6 beta engines at the time of this writing), which is enough to make the game playable on pretty much anything, if a bit visually confusing.
I do not have solid information on the performance of other card brands (such as XGI, S3, 3DLabs, etc) and sadly some of these companies have recently been bought out or shut down, so I can not recommend them for support reasons, these vendors often do not support OpenGL 2.0 shaders either (with the exception of 3DLabs).
Now a couple operating-system-specific notes:
Linux users should avoid ATI cards entirely, do not expect good performance (even from an X1800) in Linux, the ATI FireGL CAD-oriented drivers for Linux (there are no Catalyst drivers for Linux) have very bad performance, just like their Windows counterparts.
My sources tell me that Mac users should prefer ATI over NVIDIA because Apple wrote the Mac ATI drivers, however also be aware that OpenGL 2.0 shaders on Mac are suffering from Apple driver bugs (affecting both ATI and NVIDIA cards), so OpenGL 2.0 shaders may need to be turned off.