With a 7800GTX I'm fairly sure you can run ALL games at uber high settings? So go for the 19' LCD, you won't be dissappointed
There's too many people stuck in the stone age when it comes to LCD's, a 5ms white-black-white LCD can pump out 600 frames per second, refreshing the entire screen simultaneously, instead of scanning up and down like a CRT. Older LCD's were bollucks at gaming and unfortunately for them the reputation has stuck.
Then in a few months/year when you can't play on max settings anymore, then consider a second 7800GTX or whatever exists at that time
Just my 0.02c