If the address is correct, that looks like it should work.
If you want, you can download and install their gpg key
wget
http://ATrpms.net/RPM-GPG-KEY.atrpms
rpm --import RPM-GPG-KEY.atrpms
Then also include the line
gpgcheck = 1
in the yum.conf entry for atrpms.
You can read atrpms' instructions at
http://atrpms.net/install.html
There seem to be subdirectories under /rh73/, so maybe your http address isn't correct. Here is a page which includes rpms which, when installed should configure your yum or apt-get package managers to use atrpms
http://atrpms.net/dist/rh73/atrpms-package-config/
It looks like they have packages for yum 2.0 now. Perhaps you should consider updating to that. I hear it runs faster and more nicely than the original yum.
http://atrpms.net/dist/rh73/yum20/
You may need to add the others back in again, I don't know.
If all else fails, you can go here
http://atrpms.net/name/
and look for packages by name. Within each package subdirectory, the distros, including rh7.3 are offered as choices.