I want a 'ramdisk' on my server, so some temporary files can be stored in the RAM. I can create a ramdisk like this:

riksserver root # mke2fs -j /dev/ram0
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
2048 inodes, 8192 blocks
409 blocks (4.99%) reserved for the super user
First data block=1
1 block group
8192 blocks per group, 8192 fragments per group
2048 inodes per group

Writing inode tables: done
Creating journal (1024 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
riksserver root # mount -t ext3 /dev/ram0 /ramdisk

And it works! But I want this to be done every time Linux boots.
So I put this in the fstab:

/dev/ram0 /ramdisk ext3 noatime 0 0

But of course it doesn't work, because /dev/ram0 needs to be created with an ext3 filesystem first.

How can I mount ram0 as ext3 every time I boot Linux?

I am using Gentoo Linux, btw.

You could try writing a simple shell script with the commands as you've listed them (mke2fs followed by mount) and call the script from /etc/init.d/boot.local (or the Gentoo equivalent). It might be necessary to insert a sleep or wait command between the two, to give enough time for the first command to run to completion before calling the second.
However, the SuSE documentation says the boot.local script is run after boot and before going to the first runlevel. So, the script might cause the system to hang. I don't know. Be prepared to boot from cd or some such to edit the boot.local script to remove the call to the script if it does cause problems.