Simple question about adding 2nd cpu to ProLiant G5

Hello,

I'm very new to HP hardware and I got this used ProLiant ML350 G5 (tower version) for free from a friend and I'm planning to run an eCommerce website along with ubuntu server and ajenti server manager.

So far it does an overkill job for running the eCommerce with 20K products. But I'd like to increase the performance as much as possible. 

It has a Xeon 2,33 ghz E5410 installed and I'd like to add 2nd CPU.

My question is, does it have to have identical CPU or can I add another Xeon CPU with same socket profile? 

Thanks in advance.