The Service Guide (or maybe even the User's Guide) that you hopefully got with the new system should have a sysplanar diagram. I haven't worked on RS6k hardware since 1998 or else I'd just tell you. Alternatively you can call IBM Hardware and ask them, I used to work for those guys and they should be able to dig up a book/diagram if you don't have one. 1-800-IBM-SERV if they have not changed it and you'll need your serial number. The first guy/girl you talk to will be a phone jockey, so insist on speaking to a technical rep.
There might even be specs on the web, but don't trust marketing docs too much. Also, I really don't know of a way to be absolutely sure until you open the chassis and take a peek since who knows how big your DIMMs are? Unless they are listed on your purchase order.