I ordered two 2gb modules for my laptop. Turns out I would have needed only one: the t400 comes with one 2gb preinstalled.
Next: it only shows 3gb instead of 4gb (on ubuntu). Problem solved:
sudo apt-get install linux-generic-pae linux-headers-generic-pae
Next: how can I use a ramdisk - problem is that I need to copy e.g. my plone or bfg installation into that ramdisk, and need to then mount that to my old location.
mkdir -p /tmp/ram
sudo mount -t tmpfs -o size=512M tmpfs /tmp/ram/ mount --bind /tmp/ram /new/directory/path