I can confirm that Dell Intel 0XR997 D79893-017 PCI\VEN_8086&DEV_10C8&SUBSYS_A11C8086&REV_01 works perfectly fine with C2100, leaving memory untouched (so obviously BIOS issue!) Driver for the card is build in Server 2012 R2 (but it is old from 2009) Intel PROWinx64 20.7 has never driver NDIS 6.30, but the ixe62x64.inf is a mess, so the only way to install driver 2.4.36.0 is to edit the inf & then install "unsigned" driver BCDEDIT /Set LoadOptions DDISABLE_INTEGRITY_CHECKS BCDEDIT /Set TESTSIGNING ON  Latest firmware 2.3.64 for this card is available from Intel here
Came across a couple of PE C2100 (not CS2100) wityh 128 (16x8Gb) RAM They have H700 controller & one spare PCI-e slot. If I fit Intel X540-T2, then memory gets reduced to 80Gb & I briefly (too fast) error on boot about PCI not enopugh space to copy... Looks like buggy BIOS (in addition to not the best hardware)  Anybody seen anything like that with this (obviously NOT Dell) hardware?  Thanks  Seb 
PowerEge C2100 memory reduced with Intel X540