John,
Don't buy anything yet. What you can do will be based on whether you have a version 1 or a version 2 of that workstation. There was a significant change between the two, and what processors you can upgrade to is based on that as a first step.
Go into BIOS and look up the Boot Block Date, which is available under the first tab. There are limitations on what processors you can use also based what HP certified for the version you have. Let us know....
Your current processor, the E5-1620, is a quad core running at 3.6 GHz and has 10 MB cache, and is rated to run at 130W max TDP. It has a single sSpec code of SR0LC, which can be bought used off eBay for about $40.00 USD now. You can do better than that, but there are limits...
Scott