I recently bought 8GB RAM for my laptop and thought it might be interesting to try and get Linux running entirely from a ramdisk. I modified the initrd (/boot/83-mount.sh and /boot/84-remount.sh) to mount a 6GB ramdisk to /root, mounted the local harddisk to another directory and then simply copied everything with 'cp -rxa --no-preserve=timestamps' (the '--no-preserve=timestamps' was necessary since otherwise the lxdm window manager would not start; dunno why) from the local harddisk to the ramdisk. Then I unmounted the local harddisk and modified the fstab accordingly.
Everything went fine (besides that strange behavior with the timestamps), however compared to running Linux from the local harddisk, which is a Kingston V+100 SSD, the entire desktop environment runs much less snappy. For example starting Firefox takes about 3.0sec when running from SSD while from ramdisk it takes over 5.0sec; starting Gimp takes about 4.0sec with SSD and 6.5sec from ramdisk. Also, the start of lxdm login screen and then the desktop screen after login is much much slower when running from ramdisk.
To simulate the loading of the window manager and applications and check if this slow behavior of the ramdisk might be due to a bandwidth bottleneck I created a 2GB file filled with /dev/urandom on the SSD. Copying this test file from the SSD to a ramdisk takes 3.6sec (simulating the loading from SSD), while copying this file from one ramdisk to another ramdisk (simulating the loading from the ramdisk) takes 3.5sec. So, the slow startup when running from the ramdisk does not seem to be due to a bottleneck of the bandwidth.
I am running openSUSE 11.3 with 2.6.37 kernel on a ThinkPad Edge 13" with Intel Core 2 Duo SU7300 processor and 2x 4GB Kingston Value RAM DDR3-1333 CL9.