ESXi works best with 4Gb IIRC?
A VM installation of Smoothwall is possible, works and is supported but, my knowledge of VMs is non-existent. I'd prefer my firewall to be on 'real iron' rather than a VM
...and n hours of tail chasing
Just waiting to hear back from HP email support on what mobo it uses and whether they can confirm, in writing, DDY's comments above.
I'm still open to DIY suggestions though