@Pythagor: the DELL 7920 is available as a tower and as a rack system, up to DUAL processor setups. I have read conflicting information in the rack documentation. On the one hand that certain slots will not work if no 2nd CPU is installed, but then also that with one CPU all slots would work.

Such workstation systems are usually more complex than other consumer systems and require more intensive manual reading and careful preparation. I read there about PCIe card and slot priorities, etc. Not so easy to "digest".

Therefore I can only strongly recommend: in order to get started faster, you should contact DELL support directly and clarify with them how the cards can best be equipped depending on the rest of the system setup, so that it works properly afterwards.

The RME "HDSPe MADI" PCIe card has no high demands for a PCIe slot: PCIe 1.x, 1 PCIe lane, 2.5 Gbps line speed.