I did run my desktop with 1G RAM, using tmpfs for /tmp and /var/tmp for a while, without any problems. Of course it depends on your configuration and use. If you use a heavy desktop environment and do large images in Gimp, it may be a problem.
Here is my configuration for tmpfs (in /etc/fstab)
# Shared memory filesystem:
tmpfs /dev/shm tmpfs defaults 0 0
tmpfs /tmp tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0
tmpfs /var/tmp tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0