my system (CentOs5.3) became erratic after i tried to change wholesale the ownership of the /FS. is it possible to change ownership or rwx permissions of files in linux? what is the safeguard available to preserve the consistency of the program files in linux against such an attempt by su?

Changing ownership of / is not a good idea. If you are placing private/secure data then the preferred way would be to use an encrypted filesystem that only you have the key for. You can create a folder say /home/user/<username>/ and change the ownership of that to whoever, however anything with root permissions can view anything within the filesystem. However An encrypted file system's out can only be veiwed in it's encrypted form without the private key and so encryption sounds like the way I would suggest.

So you could place it down as /home/user/<username>/private/ with the private directory being the root of the encrypted filesystem. However I could have misunderstood what you meant.

This page shows how to fix the perms and ownerships for you.
If you want to change the default perms anywhere other than your personal home dir, you really need to do some reading up. The installed defaults are pretty good.
As you are using CENTOS, I recommend these manuals:
http://www.linuxtopia.org/online_boo...ide/index.html
http://www.linuxtopia.org/online_boo...ide/index.html