I run the same card on my main screen, I didn't notice any major difference.

Couple things,

There is a fairly recent Nvidia driver update from April 2nd. Geforce 182.
A quick scan of tech support forum shows more ATI complaints than Nvidia. Not really helpful but best to see if there are a lot of 200 series posts related to 3.1.

One odd Bliz post has me wondering how much they messed around with textures in the game.
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Display issues

1. Other character appears mostly black. Shoulders, helms, weapons, and cape are excluded from being black.

This issue seems to be from a new system that we put in place. To disable it, add this to your config.wtf file:
SET componentCompress "0"

You can also enter this command in your chat to set it:
/console componentCompress 0
Hit enter, then restart the game.
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The fact that you turned everything off and saw only a tiny improvment... means it will be a bit harder to troubleshoot if the driver update doesn't fix it.

More system info might help. Are the lesser systems on Nvidia as well? All on XP or Vista, 32 or 64...?