/tmp on ram write denied

[gosha]

Hello,
I followed the instructions in the fstab manpages and some tutorials on line to put my /tmp on ram. I'm trying this because my macmini has a hard disk spinning at 5400 and I have far enough ram (1g), so this might speed up things especially while browsing the web (am I right?).
It works fine if I do things as root, but as normal user (in the wheel group) I get a permission denied, cannot write on /tmp. What permission am I missing?
And, by the way, is it really worth doing this?
tks