Now that you've seen that video, you should look at the Throttlestop Overclocking thread where the Dell T3500 is very popular. 6C/12T 4+ GHz speeds and 48GB of 3 channel DDR3 1333 RAM can be more than enough, and very cost effective for most applications.
Myself and a few others have used Throttlestop software and unlocked CPUs to overclock OEM locked BIOS PCs. Throttlestop was designed for underclocking, and undevolting laptop computers to extend battery life. It also allows full control of Voltage and multiplier on unlocked CPUs. There is a...
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We haven't succeeded in overclocking the T5500 single CPU. IMO the best use for the 2 CPU T5500 is if you need virtualization for several VM applications running at once.
Depending upon the application you're running, an X5687 4C/8T may give better performance in the T5500. They'll go almost 3.9GHz in a single threaded situation, but can run 8 threads if called upon to do so. They aren't very expensive due to the unpopular 4 core architecture. 2 of them is also the fastest 2CPU option in the T5500 giving 8C/16T capability.
Here are the userbenchmark.com benchmarks for the T3500. You can look at the CPU scores and see what the various setups can do. There are a few overclocks in there. The OC speed isn't always picked up in the text, but the CPU% score, and the ranking next to other similar CPUs can reveal them.
Here are the ones for the T5500 also
The CPU% rankings for the X5xxx series CPU can be misleading because single CPU, dual CPU, and overclocks on unlocked MB are all lumped in together. Not the performance scores, just the ranking against the same X5xxx CPUs.
For instance an X5690 in a T3500 may show 40% ranking against other X5690 due to overclocks, and dual CPU setups using that CPU.
Here is an example of a T3500 scoring 71% CPU but not showing the OC except for the 96th % ranking for that unlocked CPU.
Other 6 core T3500 make around 60% CPU level, and the 4 core X5687 goes 63%. Possibly tweaked to run Turbo on all cores.