Assuming I found your comp. (http://www.pcworld.idg.com.au/review/dell/optiplex_960/277558/specs/)
Then you top out the I-3x slower than skylake time to buy a new rig cpu
Intel Core 2 Extreme QX9770 Quad-Core Processor, 3.2 GHz, 12M L2 Cache, 1600MHz FSB, LGA775
https://www.amazon.com/Intel-Extreme-Quad-Core-Processor-1600MHz/dp/B0013DMTAW/ref=sr_1_1?ie=UTF8&qid=1468811116&sr=8-1&keywords=Intel+Core2++qX9770