Install the fastest processor on the CPU support list for your computer on the dell website? If you can get a quad, get a quad. And if you have problems finding the CPU support, link me to support for your computer on the dell website.
according to dell site my optiplex 330 support only core 2 dual processors, like the e8400,
the optiplex 330 has the intel g31 chipset that supports also quad processors, so why dell don't recommend those too ?