To run Apple's Apache binary as a 32-bit process on a 64-bit Intel Mac, you can take one of two approaches:

Since the first is destructive and the second is not, the second option is recommended.

To modify the Apache launch commands, make the following changes:

HTTPD="arch -i386 /usr/sbin/httpd"
    <string>/usr/bin/arch</string>    <!-- new line -->
    <string>-i386</string>   <!-- new line -->

To verify that your change worked, start Apache and run ps -el | grep httpd. The "F" column should contain numbers that do NOT end in 4.