Here I have to find three details
1.Disk - details about disks present in the server (ie how many physical disks and size of the disk)
2.CPU - No of processors
3.Memory - Memory installed in the machine
If it's an ESX3.5, you probably have a Service Console and can try using some standard RHEL commands.
On the other hand vSphere client can definitely show you number and type of CPUs and amount of RAM installed.
Determining number and size of HDDs can become a bit more complicated if you have RAID configured. Then I would look at the machine physically, and if that's not possible - google for RAID management agent options available for your controller and ESX3.5.
Hope this helps.
Please be specific with your requirement
Do you need the details for a Physical server, ESX hosts or VM's?