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The 6800 does some things better. The x800 does some things better. The xt is slightly better overall. They both kick ass. I don't think apple sells the x800 though.
 
I agree with the above posts. Although the NVIDIA GeForce 6800 Ultra DDL can power two 30" Apple Studio HD Displays whereas the ATI RADEON X800 XT cannot. Most people do not find this a concern.

I would go with the NVIDIA GeForce 6800 Ultra DDL if you are purchasing a new Power Mac G5 or wish to run two 30" HD Displays.

I would go with the ATI RADEON X800 XT if you already own a Power Mac G5.
 
The 6800 is also a big sucker. It's so long that it blocks a PCI slot. Not to mention, it's been out a while and is becoming a bit long in the tooth. I'd go with the X800 hands down.
 
Basically, the 6800 will perform better in most games (by a hair), while the X800 will do better in GUI stuff (again, by a hair). Basically it comes down to which you want, and ATI has a MUCH better driver set, so I think thats a no brainer. Plus, as others said, it doesnt use as much power (think heat + fan noise) and doesnt block a PCI slot.
 
indeed both are fast. i am more an ati fan tho. like crazyeddie said, they have way better drivers. and u can use the ati display utility which can be a handy tool.
 
If and when I purchase a Power Mac G5, it would be the nvidea 6800. Because that is the one that Apple sells and is capable of running 30" Display. It does make it hard when something is evenly matched. Continues to drool.
 
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