You can check the HCL, but it appears to be supported
Partner Name Model CPU Series Supported Releases
HP ProLiant DL380 G3 Intel Xeon MP (Cranford/Potomac) Series ESX 3.5 U5, ESX 3.5 U4, ESX 3.5 U3, ESX 3.5 U2, ESX 3.5 U1, ESX 3.5, ESX 3.0.3, ESX 3.0.2 U1, ESX 3.0.2, ESX 3.0.1, ESX 3.0
You may be able to do some things outside VMware but probably not much. You can retrieve some things from ILO directly. You can try SNMP to ILO but you will need an SNMP server to capture data.
The HP install won't make a difference. The server is TOO old for anything other than a test machine or a very light load. You will not get the monitoring that you want. Even a G4 didn't get all that much more information.
on the hp stie it says you can have a look at this and tell me if this will work.
No. ESX only in spite of what the installation instructions say.
The HP Insight Management Agents version 8.3.0 for ESX 3.x supports VMware ESX 3.5 U4 and VMware ESX 3.5 U5 versions only.
Beware of fan speed issues on DL and esxi.
ESXi does not have the HAL to control the fans so they just run at full blast.
Remember to back EVERYTHING up before you change ANYTHING
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If you have passed the 60 days since you downloaded ESX(i) 3.5 then you can contact your VMware sales rep and ask for an eval license. You can also try calling VMware sales. The least expensive license for ESX would be the vSphere Essentials license that you could downgrade to 3.5.