I am now mixing prototype.js and xajax...! Because the refresh of the element in the right side does not need accesses to the server (can be cached on the client side) - this is using prototype.js, and because I do not know how to dynamically change the height of the iframe in the middle w/o xajax - I searched how to do that with prototype.js, maybe this is obvious but well I did not found.
Not sure the caching in IE works for the moment, I might symply cache the pages in memory with a global js variable, since the links on the right side are in very small html files!