I've instal.led Proxmox 3.3 on a HP DL360 Gen9. I had no problems, just create a RAID1 on the Hardware RAID controller, setup the system to boot in Legacy Mode and install Proxmox. It worked perfectly.
I've been trying to install Proxmox 3.3 (and Debian, and Ubuntu...) on a HP DL 180 Gen9 and it's been more complicated. It has an SmartArray Software controller (B140i) and it's not supported on Linux so first of all you have to configure the system to use "SATA AHCI MODE", after that, you can boot on Legacy Mode and install Proxmox, but now the problem is that the fan are working all the time at 58% and the server is very noisy. I've been talking with HP Support but they don't support Proxmox, nor Debian, nor Ubuntu... Just Vmware, RedHat, SuSE.