I appreciate your comments, thank you.
I uninstalled Snow Leopard and with the previous OS the dual mode seems to work fine. All I needed to do is to deactivate the OpenGL hardware acceleration in Photoshop, and images now display correctly on both screens (NEC 2690 and Macbook Pro) regardless to which is the primary display. Meaning that the extreme oversaturation has disappeared when images are displayed in Lightroom or Photoshop.
However - and this is where I would appreciate your advice - there is still a grade of 'oversaturation' (or extra colour richness) present in Lightroom or Photoshop when I compare how images appear on the Macbook Pro. I suspect this where wide gamut display differ from sRGB ones? As mentioned in my earlier post, I don't have any familiarity with wide gamut monitors, so I didn't know what to expect. My idea was to make use of the aRGB setting in my camera and of the possibility of editing in aRGB or in prophotoRGB in Lightroom and Photoshop.
But at present I mainly work for the web (I don't presently own a wide-gamut compatible printer) and not being aware that what I see on a wide gamut is NOT what I get after editing and uploading to the web - I'm afraid I made a mistake by purchasing a wide gamut monitor. I tried the sRGB setting on the NEC2690, but to my eyes the results weren't satisfactory, colours looked very pale, lifeless - compared to how they appear on the Macbook Pro's display (I tried various colour profiles in the sRGB setting, images are pale in profiles that were created for sRGB and (understandably) very pale and dark in profiles created for wide gamut).
In theory I still have two days to return/exchange this monitor. Do you think that I would be better off with the NEC2490 for webwork? I understand the 2490 is an sRGB monitor, so I expect colours would be very similar in Photoshop and after export on the web?
Another question is - is there any substantial benefit in working on a wide gamut monitor in prophoto setting then uploading the image as a JPEG in sRGB? Or shooting/editing in sRGB on an sRGB monitor could in theory lead to the same results?
And if there is any difference, on your opinion is it worth hanging on the wide gamut monitor with the discomfort of not getting what I'm seeing (and by monitoring things for sRGB output on the Macbook display)?
As you can tell, I'm completely new to this (I hope I didn't mix up things/terms too much :-) I've only used my Macbook Pro for editing before. I can't afford right now to buy a second monitor - and I'm not really sure either I really need it, so the question is really, based on what I've said, on your opinion shall I stick with the 2690 or exchange it for a different model (such as the 2490)?
Thanks for any support you can give,