Jorg\Flash,
From the User's Guide : http://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_poweredge/poweredge-r515_Owner%27s%20Manual_en-us.pdf There are only 2 channels per processor, page 106.
The 16GB RDIMM's will work in the white lever slots (1,2) or in all 4, but not mixed with other RDIMM's. Since 32GB RDIMM support has been added as well, they must also be in the slots with white levers, and only allowed capacity is 2 per processor, for a total of 128GB. You may be able to add the 2 additional 8GB RDIMM's to slots 3,4, but you may get the memory is not optimal error at startup; it may continue to POST or it may halt. If it halts, then the 8GB RDIMM's would have to be removed.
Regards,