Poona,
First of all there is no difference between the is50 and the is55 except for the color. The 55 is black vs the 50 being silver. The only difference is that the newer 55 has a retail price in the USA of $1300.00 which is a little less than the previous is50.
As to the comparison of the 640 vs the 50, the major differences are that you can program your own sounds on the Korg and the Sequencer has better editing functions. You can individually change anything one note at a time on the Korg and you can record in real time or step time including realtime overdubbing or replace mode. Basically, all the features of a more pro Korg keyboard are on the is50.
Now, here is my opinion. The Yamaha has easier to access functions. There is a 0-9keypad and a data entry wheel and the display is much clearer than the Korg. On the Korg I dislike the rubber buttons which tend to stick if you push them down on one side, and it is very difficult to know which sound or style you a choosing since there is no name showing a list on the screen. You must have a chart of paper next to you or memorize every style and sound in the keyboard. The Yamaha lets you search by catagory or scroll through a list of all sounds and styles. I also like the sounds better on the Yamaha, especially the acoustic piano. One other big difference is that on the Yamaha, you can record your own user styles and on the Korg you can only load user styles from a floppy disk. This becomes limited if you cannot find the kind of styles you like to play.
You probably wont like the sequencer as much on the Yamaha because you must put a floppy disk in to record and everytime you finish recording a track you have to save this to disk in order to hear it.
I would say it will be important what features you need to have and make you decesion based on your needs.
George Kaye
Kaye's Music Scene
Reseda, California