I have a dell t7500 with dual x5680’s and in my experience over clocking a x5660 200mhz doesn’t do a whole lot for these chips only when you overclock them to 4.2-4.5ghz that they start becoming competitive to newer chips, so I’d say go with the x5680’s , I have mine paired with a gtx 1070 and in 1080p the CPUs bottleneck the 1070 but in 4K the gpu ends up being used a lot more and becomes more worthwhile. Just my two cents