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An overclocked 3770k ($300 retail) running at 4.5 GHz will be faster in Ligthroom than a Xeon octocore E5 priced at $2,000. Another issue is that these systems are built using budget OEM components, and quality can be iffy.
Quality can be iffy in any computer. Per my observation, price is no guarantee for the quality. My daughter's Mac Pro failed three times in four years, while her 10-year old Compaq laptop that was dropped and spilled on more than one can count, still works just fine.
As you found out with the MacPro, price alone does not a guarantee quality (Apple uses OEMs like Foxconn and Delta to build their parts, just like Dell/HP/Compaq. Apple just marks up their prices a lot more than the others). However, one thing you can be sure of is that if an OEM is selling a motherboard to Dell for $34 a unit, or a power supply for $19 a unit, said mobo and PSU cannot be built to the same standards as the mobo Asus or Gigabyte sells for $200 to $300 or the Platinum PSU Seasonic sells for $120. A good board from Asus or Gigabyte (or any other name brand maker) will be built using better quality components (all the bits and pieces stuck to the board), will typically include a server grade Intel LAN chip, an upper end sound chip from Realtek or Creative, solid Japanese caps/power phases and VRMs that deliver the power to the CPU, chipset and perpherals, more features (additional USB3.0 ports, SATA ports, raid) and so on. This is not to say that the $34 board that Dell puts in their machine is defective, or that it will not function as designed over the life of the computer, it is just a lower quality part with fewer features built to a lower specification. If you build your computer, the kep is to do your research and figure out which parts have the best reputation.
As about overclocking, prior to buying computer, I spoke to my friend, Intel system architect. His recommendation was to get Ivy Bridge, good motherboard, SSD with the fastest read/write time, lots of memory, and not to bother with overclocking. Don't remember what exactly was his argument, but the main idea was that even if the processor speed appears higher, you will actually gain very little in overall performance.
If you overclock the CPU, the processor speed does not just APPEAR higher, it IS higher. If a process is not bottlenecked by I/O (disk/ram/PCIe bus), overclocking can get you signiifcant gains. If encoding a video stream takes 10 mins on a stock processor, you may be able to cut the run time down to 7.5 mins or less with overclocking. Spend a few minutes looking through the benches on the page linked below as they illustrate the gains you can achieve by overclocking your cpu:
Manufactirers like Dell lock their BIOSs to prevent overclocking because their hardware is built to minimum specs based on cost and will not safely handle overclocked demands well. If you buy from a place like Puget or build your own, you set the speed based on what you are comfortable with.