thanks.I installed using alternate cd(debian-installer) already.
The Idea of copying to ram was not resolved.But ,Debian Installer in alternate cd by default copies to RAM(/dev/tmpfs).I will experiment with live cd iso copied to ramdisk at a later time.
With Debian Installer(Which is text/ncurses based) ,it asks to Mount Ubuntu CD when booted from hard disk iso.What I did was ,to symlink(symbolic Linking) Lucid Alternate cd ISO to /dev/sr0(Which is the device file for dvd drive) .Then If I ask debian-installer to retry to mount cd ,it got mounted!
ln -sf /yourubuntuisodirectory/ubuntu.iso /dev/sr0
After that the installation was smooth.ofcourse ,I had already formatted ext4 partition for Lucid install.but, partman partitioner is available in the installer.
^^^^of course!it is a crude method.moreover udev can overwrite the changes(umounting iso).But ,for the sake of iso install on *same* partition ,it worked.
Moreover ,inside debian-installer ,it can mount lucid desktop cd also(it searches).But ,I never tried.
I am interested in to getting know how to copy iso to RAM DISK.I hope someone will answer this.