Yes the 690's take 2 people to lift out of the box (or at least one Governator). They are built tough too. Big honking metal and little plastic.
RE: Power; Yes Jeremy you are right about the power issue. I forgot to point out the power wires coming off of the riser boards. They need one connection per riser (there are 2 in the picture).
I did a quick search on Google and found a couple of different memory riser cards. Apple (for the MAC Pro), Dell, HP, IBM, all sell riser cards for some of the systems they produce. I would surmise that all of them are different but would need to verify that by testing them.
On a different note, I am getting ready to put together a couple different clusters. One for a web server, one for a visualization cluster (2+ graphics nodes with one master controlling the output of each node). If there is enough interest I will post the step by step procedure with screen shots of the install. (at least for one of them, then annotate the differences in the two) I will be using the Rocks Clusters software.
http://www.rocksclusters.org
This software is based on CentOS which is based on redhat (not sure of the version), but you can actually use ANY distro you want.