Well, you'll improve performance just due to RAM being faster then disk.
You'll want to create the RAM disk first;
# mkdir /tmp/ramd
# chmod 777 /tmp/ramd
# mount -t tmpfs -o size=512M tmpfs /tmp/ramd/
(be sure to add it to fstab as well)
Then modify your PHP.ini;
session.save_path="/tmp/ramd"