It does look like we have 32GB RDIMMS that are quad ranked now. It looks like it is the same DIMM as the LRDIMM but presents itself as a quad ranked RDIMM rather than a dual ranked LRDIMM.  Quad ranked DIMMS only support 16 total DIMMS, 8 per processor, so this configuration will not work, as it uses 24 DIMMs. You would have to use 16X32GB to reach 512GB.  The Configuration would be A1-A8 and B1-B8 ftp.dell.com/.../poweredge-r620_Owner%27s%20Manual_en-us.pdf page 48 of the manual shows the configuration you are needing. 
Hello, We have a R620 with Dual (Intel(R) Xeon(R) CPU E5-2665 0 @ 2.40GHz) at the moment they have 12x 16GB DIMMs wondering if we can add further 12x32GB DIMMS to this server so the configuration will be A1-A12 = 12X 16GB B1-B12 = 12X 32GB. Thank you
Poweredge R620 Mixed capacity RAM inquiry