Unfortunately, mixing memory modules that are not exactly alike rarely works.
The modules need to be:
Of identical manufacturer, capacity, bandwidth, and type ( 1-sided or 2-sided) and in the proper slots.
A smart way to upgrade is to purchase from a memory manufacturer that guarantees compatibility with your desktop PC. I suggest Crucial.com as a good source for these. The downside in your case is that PC is old.The memory is no manufactured by Crucial.
Take a look at the search I did for upgrade memory for your workstation in the following link.
https://www.google.nl/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=hp+workstation+xw6200+me...
Look in the PC's Maintenence & service guide to see which memory modules are supposed to be used. You may be able to find them second hand.
The RAM settings are not in the BIOS of a non-enthusiast PC's BIOS.
The primary and secondary memory timing settings are in a coded file in the BIOS provided by the PC's manufacturer, in this case, HP.

I am not an HP Employee.