About Ramdisks and RamFS
One major improvement of Plume over LTSP resides in it's far smaller memory footprint, due to extensive usage of new features in the 2.4 Linux kernel revisions. The main one is RamFS.
The common way of storing files where read-write access is needed is to put them into a Ramdisk, a fixed-size(at boot time) solution for creating a virtual block device for storing a filesystem. That means that the way to use them is to create a Ramdisk, format it (create a filesystem on it with mkfs) and filling it with the desired files.
RamFS is far better, because it is dynamically allocated (size varies dynamically as needed) and is a real filesystem, not a fake device on which to put a filesystem. You do not waste RAM at all. The filesystem only uses the space needed to store the files on it. You don't have to specify a size at all.
TmpFS is a variant of RamFS, meant to use also swap if available, which is not the case in Plume.
You can find more information about RamFS and TmpFS and their differences in the kernel sources Documentation/filesystems/tmpfs.txt file.