for refresh rate in XP that is purely microsoft "intentional stupidity". you need a 3rd party refresh rate fix. I use refresh force, but there are others.
as to radeon performance w/ Dom II, well, it is pretty funny
I don't know what the explanation is. I would expect some strange thing in the engine that Dominions used. For a long time nVidia was so dominant, a lot of devs only bothered using nvidia cards to develop on. This was, of course, actively encouraged by nvidia reps, who also pushed their own nvidia-specific OGL extensions. Its possible that this happened during the creation of the original graphics engine upon which Dom II is based .
Re: Display & Other Problems
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