Hi Kai,
Thanks for your excellent explainations It was really very very helpfull..
I. I have changed the Crystal 33pF arrangment as per your suggestion.

II. I have changed the arrangement of PS/2 keyboard connector X1 traces as per your suggestion the power plane has become more solid by applying your suggestion.

III. I will try to implement chokes in my design.

IV. I have noticed a big layout mistake! If you connect a number of digital lines from one board to a second, then you must also connect ground lines for the individual ground return currents with this certain cable!!!
These ground lines must be as close as possible to individual digital lines you transmit from board to board. Only by this, the formed loop (digital line and its ground return current) is kept minimal. If you do not provide such a close ground line for each digital line, then heavy radiation and susceptibility to radiation will be the consequence!
It's a good idea, to use a flat cable for this purpose, where each second, or at least third line is a ground line. All these ground lines MUST be connected to ground at each board!

Certainly I will implement this suggestion as well but just wanted to know is this required even if my signal frequency is very low say less than 100Hz.

Thanks & Regards,
Prahlad Purohit