For the MDS servers you would like to have as much memory as you can afford so that you can cache as much as possible in the MDS server. But, the cost to jump to higher GB DIMMs can be prohibitive. We routinely use 48 GB, or 24GB per core for MDS servers and this combination works very well. If you can afford higher amounts of memory then that should be even better.
A detailed discussion of Lustre design considerations can be found in the whitepaper LUSTRE WHITEPAPER DOWNLOAD http://wiki.whamcloud.com/download/attachments/425994/Lustre+OSS+and+MDS+server+node+Requirements+white+paper.docx