I have a G4 with Geforce FX 5950 Ultra, runs the Doom3 Timedemo full options at a lovely 75fps 1024x768
Most Macintosh are shipped with Radeon 9200 or Geforce FX 5200.
I'm still not sure why...
This said the Mac is also far far far far far far far, more expensive than it should be.
To get a PC Equal power to a $2,700 Mac, your looking at around $1,000 max;
And the X-Box can do the games for cheaper because the hardware is done in bulk and cheaper to make. Remember the X-Box was 1year old technology when it hit the market, and has now been on the market for almost 3years. So your looking at pretty ancient technology in computing terms.
The graphics card really in newer games, is the be-all and end-all of your power; you have a crap card no matter how powerful your processor is, it ain't gonna help.