In /etc/fstab, use the following lines.
shm /dev/shm tmpfs mode=1777,nr_inodes=1M,size=X 0 0
/tmp /dev/shm none bind 0 0
The maximum size I suggest for the size of X is 75% of your total RAM. This will still give you enough space when your /tmp fulls up during use. Also movies that mplayerplug-in downloads will be a lot faster when playing back the videos even when SDRAM memory used. Since the permission for /tmp can be use by anybody, you will have to secure your setup to the users that you trust. You may want to use AIDE or tripwire to keep an eye on /tmp.
I only recommend this setup if you are using ECC memory. There is a higher possibility of data corruption when not using ECC memory.
The following link that I found a few years ago but still is good.