Without a GPU power cable overclocking puts the MB PCIe slot at risk of damage. So I would take what you get from the GPU and be happy with that. Dell PSUs are typically under rated. They don't sell them by the Watt. I don't have tests for all of them but the old Optiplex 305W typically made 400W before shutting down safely. It's just about impossible to overload one. They don't have any extra cables to connect anything to. The newer high efficiency ones might not have as much headroom, but I would expect them to make their rated power reliably. The 265-275W version is just a high efficiency replacement for the 305W that powered 130W Pentium Ds back in the day. They just chopped off 40W extra that the 95W CPUs don't need...