Hi:
If you are referring to having an HP 8100 Elite CMT or SFF, you can upgrade to a first gen Intel core i7 processor, but you would also have to install a graphics card because none of the Intel 1st generation i5 or i7 quad core processors work with the onboard Intel graphics like the dual core i3 and i5 processors do.
Below is the link to the HP quickspecs for the 8100 Elite model series.
HP Compaq 8100 Elite Series (noteboox.de)
The supported processors can be found on pages 6 & 7.
Note the reference to the required graphics card for the quad cores listed on page 7.