You bought a Dell Optiplex 330 C2D computer. As stated above C2D refers to Core 2 Duo which is an older Intel CPU architecture from 2006.
Below is a link to the 330's technical guide book. Page 8 list all the CPUs compatible with the motherboard. It basically ranges from the E2160 up to the E7200; all are dual core CPUs. The E6850 is actually the most powerful CPU that can be used.
Those CPUs were released by Intel between mid 2007 through mid 2008; more or less.