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Two things:
A GT will be fine at that res - I have the same monitor and I did have a GT. However, for the extra bit of money, better cooler, more stream processors (may come in handy for physics - see other thread on here) I would buy the GTS 512Mb. At that res, overclocked it will be faster than a GTX unless you put on loads of AA in which case in some games the GTX might be slightly ahead.
Secondly, willhub, the NEC is miles ahead of your screen. Look here:
http://forums.overclockers.co.uk/showthread.php?t=17772691
And here:
http://www.tftcentral.co.uk/reviews/nec_20wgx2_phase2.htm
You will see it is a high quality S-IPS screen which in real world tests is "more responsive than a 6ms TN Film panel" and "on par with a 4ms TN Film monitor "
Colour gamut is superb and the DeltaE colour fidility readings averaging 0.1 makes it suitable for semi professional use. And the blacks are just so deep and fantastic.
The reflective screen isn't everybody's cup of tea and you will either love it or hate it but I find it a superb monitor and makes my games and videos look superb.
Yes, it's a lot of money for a 20.1" screen but the TN film 22" monitors don;t even come close to it.