AFIK the X5600s are not unlocked. The unlocked Xeons seem to be all in the W series. The BCLK method is just a new name for the FSB method to overclock locked CPUs on unlocked motherboards. This has been the standard method of overclocking due to the prohibitive cost of unlocked CPUs. There are hundreds of forums, and unlimited posts regarding this technique.
OEMs used modified BIOS without overclocking controls, and also CPU wattage limits to keep unlocked CPUs out of office computers. Sometimes they even seem to be blacklisted. Due to warranty and tech. support concerns it's quite justified. Also office computers are a cost sensitive market and they don't have the motherboard power to support much of an overclock anyway with a few exceptions. There have been a few low powered unlocked CPUs that can run in these. But AFIK no ones tried TS overclocking yet. The low powered unlocked CPUs should be kept in mind because they may actually overclock higher due to offering more thermal head room.
Throttlestop was developed for underclocking , undervolting, and removing thermal throttling on laptops. Sometimes to increase performance, but also to extend battery life. There are several large forums dedicated to this activity. Some of them over 1000 pages long. Hidden in them are reports of unlocked CPU overclocking, mostly on laptops. I created this Desktop Overclocking thread so others wouldn't have to wade through thousands of laptop posts to find this information. But realistically it's so little known or practiced that you are very likely to be the first one to try it on any particular combination of MB, or CPU that you try. Props to the laptop overclockers out there. They have it much harder than desktop overclockers, and they dig really deep into their systems to get their results. We're talking board level hardmods, and chipset and memory controller editing. The difference between TS and other overclocking software is it's ability to raise Volatge on unlocked CPUs. SetFSB, Clockgen, etc. can raise FSB, but without control of Voltage results are seriously limited.
For this to work you must have an unlocked CPU. Core2Extremes that were $1000 are now availablefor <$80. Locked BIOS computers in the LGA775 can be had for almost nothing, and X58 systems for not much more. The other thing that's required is a motherboard that can run an unlocked CPU.
http://www.userbenchmark.com/?redirFrom=userbenchmark.com& is very useful for finding these. You can see at a glance hundreds, often thousands, of examples of just about any system. It very common to see unlocked CPUs running there on locked motherboards but no overclocks posted. The few systems that have been overclocked are easy to see because the CPU score will be way beyond those of the base clocked CPUs. The Dell T3400 is a good example. The QX9650 score 41% at base speed, but jumps up to 57% with a 4.15GHz TS overclock. Overclocked T3500s are posting scores over 100% compared to their target "modern gaming system". Thermal mods used in traditional overclocking definitely still apply here. Heatsinking The VRMs, big CPU coolers, fan and airflow mods, lapped CPUs all still apply.
The best candidates for this seem to be the workstations, or high end gaming computers, old WS being common and therefore cheap, but the gamers might be found with decent GPUs. Dual GPU support is a huge bonus. Next would be multimedia systems, and for those in dire financial straits the office computers that can run unlocked CPUs (towers much preffered due to PSU, and GPU sizes).
Other things to watch for at userbench are missing CPUs that one would expect. This can indicate a lack of chipset or BIOS support ( many Q9550, but no Q9650 for example). Sometimes only early CPU steppings are supported. Look for Xeons running, or not. With Dells you will notice an almost total lack of AMD GPUs newer than R7 series. There is a known (among Dell users) BIOS issue with AMD GPUs from GCN3 on up. This was resolved on newer Dells. But for the systems being considered here it's typical. You might see a couple running ( I've done it) but the BIOS settings will not be available at all.