Can someone confirm that ESXi5 v.5.1 works on a HP ProLiant DL320 G5. VMWare compatibility matrix shows DL320 G6. If it works, do you use any specific drivers ?
Sometimes the VMware HCL isn't just about whether something will work, but rather, does it makes sense? This server doesn't make sense...
But, ESXi compatibility really hinges on your array controller and NIC support. The chipset of the ProLiant DL320 G5 is supported. The processors may be capable. The NIC is supported. It really depends on whether you have an add-on PCIe Smart Array controller installed. The native Intel 82801GR storage controller is a "Fake RAID" controller, and won't be supported.
The details of your server matter in this case. Do you have more information?
Keep in mind that this is a low-end server... 2 drive slots, 8GB RAM maximum, one CPU... so even if compatible, the utility of this server as an ESXi host is low, given more modern options out there.