i want to know what is the different between DVI and D-SUB..

D-sub is analog video input and dvi is digital input.

Although Dvi on an LCD screen wont give you much more over the D-sub,

I myself Like Crt's for the quality in games and such there are some LCD's that could compete with a CRT, but not many. And for the price I got a 24 inch flat crt for 500.00 try finding a 24 inch LCD for that :-)
Hope that answers your questions

i m useing a CRT monitor and i want to know DVI or D-SUB is better for my CRT mon.

You cannot use DVI with your CRT it is only a D-Sub connection,

should i use the dvi for my new lcd mon or use the d-sub. the graphic card is geforce 6600gt 128

If you have the choice go for the DVI. On an LCD when you or a game changes the resolution the screen won't have to 'calibrate' and will adapt automatically perfect to the resolution.

If you are using the analog interface (D-Sub) make sure when you switch from a CRT to an LCD you lower your refresh rate to 60 Hz.in your advanced display properties. ::idea::