Hi
Unfortunately, upgrading your processor won't help you run ESXi 5.x on your DL580 G3 - this machine will only run up to 3.5.
The below link are downloads compatible with your HP Proliant DL580 G3 server. Please let me know if you need any assistance in which download will fit your needs.
VMware ESX/ESXi Server 3.5 downloads
I'd like to connect you to one of our specialists who has more details on these products and can help you find a solution that will enable you to run ESXI 5.x. I sent you a private message with some information I need in order to connect you to the right rep. Please let me know if you would like me to set this up for you.