Can you use a core 2 duo in a dell optiplex gx260 motherboard?

No, a Core 2 Duo processor is not compatible with a DELL OptiPlex GX260 motherboard. The GX260 uses the Intel Pentium 4 or Intel Xeon processors, which are based on the older socket 478 architecture. Core 2 Duo processors require a different socket type (LGA 775) and chipset that the GX260 does not support.