Sorry about the confusion on agpart, it was late
The module is CONFIG_AGP under character devices. Check the info about NvAgp in the nvidia drivers readme line 1286, I have mine set to "1" which the readme says uses the NVAGP rather than AGPART. Also try removing your agp module before starting X.
I am running at 1600x1200 24bit, I will check the other resolution after I finish recompiling garnome
Yes it is very weird that no one else has this problem. I firt noticed it back in November and I have searched the web backwards and forwards and haven't found any other posts about it.
One other idea - have you tried downgrading to 3123 and testing Option "NvAgp" "1" under device?
Interesting point about hardware vs. software emulation - I will check if mplayer tells you which it is using - you can force software scaling by using mplayer -vop scale -zoom ----check man mplayer line 1826 or so.
Thats all for now, Im curious as to whether you can get it fix and its weird that changing NvAgp worked for me and not you