Now we have migrated to two openldap (2.4.21, ubuntu lucid 64btis) farms. One with 4 nodes (xen vm with two cpu cores, Xeon E5450 3GHz, 2 GB of RAM) for the authentication database, and other with 2 nodes (with the same resources than the other) for the other database.
OpenLDAP 2.4.21 is over 2 years old. I would first suggest upgrading to a current release, given the hundreds of bug fixes since then. If you still see load issues, I highly advise reading over <http://wiki.zimbra.com/wiki/OpenLDAP_Performance_Tuning>.
--Quanah -- Quanah Gibson-Mount Sr. Member of Technical Staff Zimbra, Inc A Division of VMware, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration